The Experts below are selected from a list of 11913 Experts worldwide ranked by ideXlab platform
Jakob Magid - One of the best experts on this subject based on the ideXlab platform.
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PHOSPHATE SORPTION TO MACROPORE Wall MATERIALS AND BULK SOIL
Water Air and Soil Pollution, 2002Co-Authors: Marina Bergen Jensen, Hans Christian Bruun Hansen, Jakob MagidAbstract:Preferential flow through macropores may allow reactive solutes to travel long distances in soil. The amount of solute transported is expected to depend on the ability of the macropore Wall materials to retain the solute from the bypassing solution. From a loamy sand soil, samples of bulk Ap-horizon, bulk Btg-horizon, earthworm burrow lining, and both iron-depleted and iron-enriched materials from glossic Fracture Walls were obtained. The different soil materials were shaken for 5 min, 2 hr and 7 d in 0.01 M CaCl2 with initial P concentrations from 0 to 3.3 mg H2PO4--P L-1 at pH 5. The resulting changes in solution P-concentration were interpreted in terms of P-desorption or P-sorption. Burrow lining and bulk-Ap were poor P sorbents, especially at short contact times (5 min and 2 hr). They were unable to sorb P at concentrations below approximately 1 mg PO4-P L-1. In contrast, Fracture Wall materials and bulk-Btg were much stronger sorbents. They removed P from solutions having P-concentrations of only about 0.03 mg PO4-P L-1. The results of the study suggest that environmentally critical concentrations of dissolved Pi will be leached more easily through earthworm burrows than Fractures, and that sorption characteristics of bulk soil may deviate strongly from sorption characteristics of macropore Wall materials.
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Phosphate sorption to matrix and Fracture Wall materials in a Glossaqualf
Geoderma, 1999Co-Authors: Hans Christian Bruun Hansen, Marina Bergen Jensen, Jakob MagidAbstract:Abstract Phosphate retaining properties of macropore Wall materials may influence the extent of phosphate leaching via preferential flow. Phosphate sorption to soil matrix material has been compared with sorption to Fracture Wall materials consisting of inner iron-oxide depleted albic coatings rimmed by reddish iron-oxide enriched quasicoatings. The latter contains 15 times as much Fe oxide, 2 times as much Al (hydr)oxide and 5–6 times as much total P than the albic material, which represent the most P-depleted material in the profile. Oxalate extractable P (Po) amounts to 1 4 − 1 2 of the total P contents of the samples. Smectite predominates in the Fracture Walls and acid subsoils, but has partly transformed into hydroxy-interlayered smectite (HIS) in the upper limed soil horizons. Langmuir parameters derived from phosphate sorption isotherms show no correlation for sorption to whole soil samples and the corresponding clay fractions. The phosphate sorption capacity at a threshold equilibrium concentration of 10 μM (Padst (10 μM)) is measured as the content of Po plus the amount of phosphate sorbed during 7 days, and can be related to Alo+Feo: Padst (10 μM)=(0.0404±0.0046)·(Alo+Feo)***+3.569. The Padst (10 μM) values vary between 4 and 14 mmol P kg−1, lowest in the albic Fracture Walls and highest in the iron enriched quasi-coatings. The sorption capacity of the metal oxide free clay does not appear to correlate with smectite or HIS contents. In total, the clay fraction contributes with about 50% of the whole soil sorption capacity in agreement with 40–60% of the total soil Alo+Feo being present in the clay fraction. At a solution phosphate concentration of 10 μM, the Ap horizon is found to be almost phosphate saturated whereas subsoil horizons including the Fractures have phosphate saturations ≤50% and, hence, can strongly sorb P. The low phosphate saturation of Fracture Walls indicates that they do not act as major sinks of phosphate-rich solutions from topsoil horizons, either due to kinetic constraints, or due to a flow pattern not allowing P-rich solution to percolate Fractures.
Yves Méheust - One of the best experts on this subject based on the ideXlab platform.
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Sensitivity Analysis of Heat Transport in Self-Affine Rough Fractures
2020Co-Authors: Maria Klepikova, Clément Roques, Yves Méheust, Niklas LindeAbstract:The characterization of thermal transport in Fractured rocks is crucial for understanding numerous systems of environmental, geological, and industrial importance. Fracture Wall roughness exhibits long range spatial correlations and induces a heterogeneous aperture field, thus promoting the formation of preferential flow channels within Fracture planes. Here, we develop a modelling approach to identify geometrical parameters of individual Fractures that control the heat exchange at the fluid/rock interface and the advection of heat along the Fracture.
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Dynamics of foam flow in a rock Fracture: Effects of aperture variation on apparent shear viscosity and bubble morphology
Journal of colloid and interface science, 2019Co-Authors: Mohammad Javad Shojaei, Yves Méheust, Antonio Rodríguez De Castro, Nima ShokriAbstract:There has recently been renewed interest in understanding the physics of foam flow in permeable media. As for Newtonian flows in Fractures, the heterogeneity of local apertures in natural Fractures is expected to strongly impact the spatial distribution of foam flow. Although several experimental studies have been previously performed to study foam flow in Fractured media, none of them has specifically addressed that impact for parallel flow in a realistic Fracture geometry and its consequences for the foam's in situ shear viscosity and bubble morphologies. To do so, a comprehensive series of single-phase experiments have been performed by injecting pre-generated foams with six different qualities at a constant flow rate through a replica of a Vosges sandstone Fracture of well-characterized aperture map. These measurements were compared to measurements obtained in a Hele-Shaw (i.e., smooth) Fracture of identical hydraulic aperture. The results show that Fracture Wall roughness strongly increases the foam's apparent viscosity and shear rate. Moreover, foam bubbles traveling in regions of larger aperture exhibit larger velocity, size, a higher coarsening rate, and are subjected to a higher shear rate. This study also presents the first in situ measurement of foam bubbles velocities in Fracture geometry, and provides hints towards measuring the in situ rheology of foam in a rough Fracture from the velocity maps, for various imposed mean flow rates. These findings echo the necessity of considering Fracture Wall when predicting the pressure drop through the Fracture and the effective viscosity, as well as in situ rheology, of the foam.
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Coupled Electro-hydrodynamic Transport in Geological Fractures
2018Co-Authors: Yves Méheust, Uddipta Ghosh, Tanguy Le BorgneAbstract:Geological Fractures constitute the basic structural units controlling the flow of fluids and the transport of solutes in subsurface crystalline rocks. Fracture Wall roughness is responsible for flow channeling within the Fracture plane, which impacts the Fracture's transmissivitty [1], and can also impact the distribution of fluxes in-between Fractures of the Fracture networks [2]. The most prevalent way of computing the distribution of local fluxes in (and transmissivitty of) a rough Fracture without resorting to a full 3D flow simulation, is to use the lubrication approximation, which leads to a simple linear equation for pressure: the Reynolds equation. However, the effect of the electrical properties of the solid Walls on the transport properties of a Fracture still remains an open question. Since dissolved minerals and salts are present in the fluids, Electrical Double Layers (EDLs) form at the fluid-solid interface [3]. Hence, the occurrence of externally-imposed or naturally-occurring gradients in electrical potential and/or ionic concentration leads to significant changes in the fluid flow and solute transport as compared to flows driven primarily by hydraulic head differences. We consider geological Fractures with a realistic aperture field and explore the flow dynamics resulting from such coupled electro-hydrodynamic forcings. To this end we generalize the standard lubrication theory for flow to include a description of the coupled transport of fluid mass, solutes, and electrical current under application of fixed differences in hydraulic head (or pressure), electrical potential and concentration across the Fracture. This generalized lubrication theory is solved using an iterative Finite Volume Method.
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Coupled Electro-hydrodynamic Transport in Geological Fractures,
2018Co-Authors: Uddipta Ghosh, Tanguy Le Borgne, Yves MéheustAbstract:Fractures are very common features in subsurface crystalline rocks, where they are organized in networks of interconnected elements [1]. A number of essential mechanical properties of the rock formations, such as their mechanical strength and their transport properties (hydraulic and electri- cal conductivities), are dictated by the behavior of the Fracture networks. Within these networks, individual geological Fractures are the basic structural unit controlling the ow of uids and the trans- port of solute chemical species. Their length is distributed over a very large range, which strongly constrains the connectivity and hydraulic behavior of the network [2]. Fracture Wall roughness is responsible for ow channeling (and therefore, heterogeneity) within the Fracture plane, which, at the Fracture scale, impacts the Fracture's transmissivitty [3, 4]. The characteristic length scale Lc at which the two Fracture Walls are matched [5, 6], plays a crucial role as it is the upper limit scale for ow heterogeneities [7]. When Lc is suciently large with respect to the distance between two intersections with other Fractures, Fracture Wall roughness also impacts the distribution of uxes in-between Fractures of the network [8]. The most prevalent way of computing the transport properties and transmissivitty of a rough Fracture in an ecient way and without resorting to a full three-dimensional ow simulation, is to use the lubrication approximation, which leads to a Darcy ow type equation for the pressure, the Reynolds equation [3]. This method has been used extensively to simulate the ow [3, 9], as well as the electric current (without ow) through a rough Fracture [10]. However, the eect of the electrical properties of the Fracture Walls on the transport properties of a Fracture still remains an open question, to the best of our knowledge. Since dissolved minerals and salts are ever present in the uids inside the Fracture, Electrical Double Layers (EDL) almost inevitably form at the uid-solid interface [11], and their strength depends on the chemical properties of the rock and ionic strength of the uid. Therefore, the ocurrence, at the Fracture scale, of externally-imposed or naturally-occurring gradients in electrical potential and/or ionic concentration, can lead to signicant changes in the uid motion through the Fracture, as compared to ows driven primarily by hydraulic head dierences. In this work, we attempt to explore the ow dynamics that result from such coupled electro- hydrodynamic forcings. To this end, we generalize the standard lubrication theory for ow, to include a description of the coupled transport of uid massn, solutes, and electrical current under application of xed dierences in hydraulic head (or pressure), electrical potential and concentration across the Fracture. By invoking the requirement of conservation of volumetric ow rate, ions and electrical charge uxes, a coupled system of equations can be derived, which governs the spatial distribution of electrical potential, pressure and concentration in the bulk uid within the Fracture. This system of equations is the generalization of the Reynolds equation to the coupled transport of uid mass, solutes, and electrical charges. It is solved using an iterative Finite Volume Method to gain insight into the dynamics of the coupled transport processes, in geological Fractures with a realistic aperture field. We investigate in particular the role of the characteristic length scale Lc.
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Scale Effects in the Flow of a Shear-Thinning Fluid in Geological Fractures
2017Co-Authors: Clément Roques, Yves Méheust, Tanguy Le Borgne, John SelkerAbstract:Subsurface flow processes involving non-Newtonian fluids play a major role in many engineering applications, from in-situ remediation to enhanced oil recovery. The fluids of interest in such applications (f.e., polymers in remediation) often present shear-thinning properties, i.e., their viscosity decreases as a function of the local shear rate. We investigate how Fracture Wall roughness impacts the flow of a shear-thinning fluid. Numerical simulations of flow in 3D geological Fractures are carried out by solving a modified Navier-Stokes equation incorporating the Carreau viscous-shear model. The numerical Fractures consist of two isotropic self-affine surfaces which are correlated with each other above a characteristic scale (thecorrelation length of Méheust et al. PAGEOPH 2003). Perfect plastic closing is assumed when the surfaces are in contact. The statistical parameters describing a Fracture are the standard deviation of the Wall roughness, the mean aperture, the correlation length, and the Fracture length, the Hurst exponent being fixed (equal to 0.8). The objective is to investigate how varying the correlation length impacts the flow behavior, for different degrees of closure, and how this behavior diverges from what is known for Newtonian fluids. The results from the 3D simulations are also compared to 2D simulations based on the lubrication theory, which we have developed as an extension of the Reynolds equation for Newtonian fluids. These 2D simulations run orders of magnitude faster, which allows considering a significant statistics of Fractures of identical statistical parameters, and therefore draw general conclusions despite the large stochasticity of the media. We also discuss the implications of our results for solute transport by such flows. References: Méheust, Y., & Schmittbuhl, J. (2003). Scale effects related to flow in rough Fractures. Pure and Applied Geophysics, 160(5-6), 1023-1050.
Derek Elsworth - One of the best experts on this subject based on the ideXlab platform.
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Evolution of Shale Permeability under the Influence of Gas Diffusion from the Fracture Wall into the Matrix
Energy & Fuels, 2020Co-Authors: Jie Zeng, Derek Elsworth, Jishan Liu, Yee-kwong Leong, Jianchun GuoAbstract:Permeability is the most important property that controls the transfer of gas mass across a hierarchy of scales within a shale gas reservoir. When gas diffuses from the Fracture Wall into the matri...
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long term evolution of coal permeability under effective stresses gap between matrix and Fracture during co2 injection
Transport in Porous Media, 2019Co-Authors: Mingyao Wei, Derek Elsworth, Jishan Liu, Rui Shi, Zhanghao LiuAbstract:Understanding the long-term evolution of coal permeability under the influence of gas adsorption-induced multiple processes is crucial for the efficient sequestration of CO2, coalbed methane extraction and enhanced coal bed methane recovery. In previous studies, coal permeability is normally measured as a function of gas pressure under the conditions of constant effective stresses, uniaxial strains and constant confining pressures. In all these experiments, an equilibrium state between coal matrix and Fracture is normally assumed. This assumption has essentially excluded the effect of matrix–Fracture interactions on the evolution of coal permeability. In this study, we hypothesize that the current equilibrium assumption is responsible for the discrepancy between theoretical expectations and experimental measurements. Under this hypothesis, the evolution of coal permeability is determined by the effective stress gap between coal matrix and Fracture. This hypothesis is tested through an experiment of CO2 injection into a coal core under the constant effective stress. In this experiment, the effective stress in the Fracture system is unchanged while the effective stress in the matrix evolves as a function of time. In the experiment, the coal permeability was measured continuously throughout the whole period of the experiment (~ 80 days). The experimental results show that the core expands rapidly at the beginning due to the gas injection-induced poroelastic effect. After the injection, the core length remains almost unchanged. But, the measured permeability declines from 60 to 0.48 μD for the first month. It rebounds slowly for the subsequent 2 months. These results indicate that the effective stress gap has a significant impact on the evolution of coal permeability. The switch of permeability from the initial reduction (the first 30 days) to rebound (the subsequent 50 days) suggests a transition of matrix deformation from nearby the Fracture Wall to further away area. These findings demonstrate that the evolution of coal permeability is primarily controlled by the spatial transformation of effective stresses between matrix and Fracture.
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Role of proppant distribution on the evolution of hydraulic Fracture conductivity
Journal of Petroleum Science and Engineering, 2018Co-Authors: Jiehao Wang, Derek ElsworthAbstract:Abstract The residual opening of fluid-driven Fractures is conditioned by proppant distribution and has a significant impact on Fracture conductivity - a key parameter to determine fluid production rate and well performance. A 2D model follows the evolution of the residual aperture profile and conductivity of Fractures partially/fully filled with proppant packs. The model accommodates the mechanical response of proppant packs in response to closure of arbitrarily rough Fractures and the evolution of proppant embedment. The numerical model is validated against existing models and an analytic solution. Proppant may accumulate in a bank at the Fracture base during slick water fracturing, and as hydraulic pressure is released, an arched zone forms at the top of the proppant bank as a result of partial closure of the overlaying unpropped Fracture. The width and height of the arched zone decreases as the fluid pressure declines, and is further reduced where low concentrations of proppant fill the Fracture or where the formation is highly compressible. This high-conductivity arch represents a preferential flow channel and significantly influences the distribution of fluid transport and overall Fracture transmissivity. However, elevated compacting stresses and evolving proppant embedment at the top of the settled proppant bed reduce the aperture and diminish the effectiveness of this highly-conductive zone, with time. Two-dimensional analyses are performed on the Fractures created by channel fracturing, showing that the open channels formed between proppant pillars dramatically improve Fracture transmissivity if they are maintained throughout the lifetime of the Fracture. However, for a fixed proppant pillar height, a large proppant pillar spacing results in the premature closure of the flow channels, while a small spacing narrows the existing channels. Such a model provides a rational means to design optimal distribution of the proppant pillars using deformation moduli of the host to control pillar deformation and flexural spans of the Fracture Wall.
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Evolution of Friction and Permeability in a Propped Fracture under Shear
Hindawi-Wiley, 2017Co-Authors: Fengshou Zhang, Yi Fang, Chaoyi Wang, Derek Elsworth, Xiaofeng YangAbstract:We explore the evolution of friction and permeability of a propped Fracture under shear. We examine the effects of normal stress, proppant thickness, proppant size, and Fracture Wall texture on the frictional and transport response of proppant packs confined between planar Fracture surfaces. The proppant-absent and proppant-filled Fractures show different frictional strength. For Fractures with proppants, the frictional response is mainly controlled by the normal stress and proppant thickness. The depth of shearing-concurrent striations on Fracture surfaces suggests that the magnitude of proppant embedment is controlled by the applied normal stress. Under high normal stress, the reduced friction implies that shear slip is more likely to occur on propped Fractures in deeper reservoirs. The increase in the number of proppant layers, from monolayer to triple layers, significantly increases the friction of the propped Fracture due to the interlocking of the particles and jamming. Permeability of the propped Fracture is mainly controlled by the magnitude of the normal stress, the proppant thickness, and the proppant grain size. Permeability of the propped Fracture decreases during shearing due to proppant particle crushing and related clogging. Proppants are prone to crushing if the shear loading evolves concurrently with the normal loading
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Evolution of permeability in a natural Fracture: Significant role of pressure solution
Journal of Geophysical Research, 2004Co-Authors: Hideaki Yasuhara, Derek Elsworth, Amir PolakAbstract:[1] A mechanistic model is presented to describe closure of a Fracture mediated by pressure solution; closure controls permeability reduction and incorporates the serial processes of dissolution at contacting asperities, interfacial diffusion, and precipitation at the free face of Fractures. These processes progress over a representative contacting asperity and define compaction at the macroscopic level, together with evolving changes in solute concentration for arbitrarily open or closed systems for prescribed ranges of driving effective stresses, equilibrium fluid and rock temperatures, and fluid flow rates. Measured Fracture surface profiles are applied to define simple relations between Fracture Wall contact area ratio and Fracture aperture that represents the irreversible alteration of the Fracture surface geometry as compaction proceeds. Comparisons with experimental measurements of aperture reduction conducted on a natural Fracture in novaculite [Polak et al., 2003] show good agreement if the unknown magnitude of microscopic asperity contact area is increased over the nominal Fracture contact area. Predictions of silica concentration slightly underestimate the experimental results even for elevated microscopic contact areas and may result from the unaccounted contribution of free face dissolution. For the modest temperatures (20–150°C) and short duration (900 hours) of the test, pressure solution is demonstrated to be the dominant mechanism contributing to both compaction and permeability reduction, despite net dissolution and removal of mineral mass. Pressure solution results in an 80% reduction in Fracture aperture from 12 μm, in contrast to a ∼10 nm contribution by precipitation, even for the case of a closed system. For the considered dissolution-dominated system, Fracture closure rates are shown to scale roughly linearly with stress increase and exponentially with temperature increase, taking between days and decades for closure to reach completion.
Marc Ulrich - One of the best experts on this subject based on the ideXlab platform.
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Slab-derived origin of tremolite-antigorite veins in a supra-subduction ophiolite; the Peridotite Nappe (New Caledonia) as a case study.
International Journal of Earth Sciences, 2019Co-Authors: Dominique Cluzel, Philippe Boulvais, Marion Iseppi, Didier Lahondère, Stéphane Lesimple, Pierre Maurizot, Jean-louis Paquette, Alexandre Tarantola, Marc UlrichAbstract:Hydration of mantle peridotites provides information on the exhumation history and on the fluid regime accompanying exhumation of these rocks (reservoirs involved, fluid/rock ratios, temperature of interaction). Highly depleted harzburgites and dunites of the Peridotite Nappe of New Caledonia are crosscut by Fractures, which have been pervasively serpentinized, producing lizardite, brucite, magnetite and minor chrysotile in a near-static environment, probably by sea water circulating in cooling joints. This event is generally referred to as “primary serpentinization”. In a next step, already serpentinized joints were re-opened to produce tension and shear cracks sealed by higher-temperature synkinematic fibrous minerals. Locally, tremolite-bearing veins and pockets, which do not display evidence for void infill, were generated by metasomatic replacement of the Wall rock peridotite. Most veins only contain fibrous antigorite but some display tremolite-antigorite intergrowths or even pure tremolite. The latter rocks yield high contents of Ca and incompatible elements, which contrast with the overall depletion of the peridotite host rock and suggest contribution of an external source. Whole-rock geochemical and isotopic features (87Sr/86Sr, 18O and H) suggest that antigorite veins, which bear highly radiogenic Sr isotope signatures, were strongly influenced by fluids emitted by the subducted slab and associated sediments. In contrast, the geochemical and isotopic signatures of tremolite-bearing rocks suggest a genetic link with Early Eocene supra-subduction dykes and fluids that leached them. Calcium, strontium and REE-bearing oxidized fluids reacted with already serpentinized Fracture Wall rock as shown by fibre nucleation, chromite alteration and high Cr contents in tremolite. The bulk of syn-tectonic fluid-rock interaction associated with shear and tensile Fracture development probably occurred at the onset of Eocene intra-oceanic subduction, when the buoyant lower plate (the South Loyalty Basin) obliquely forced its way beneath the nascent Loyalty fore-arc. Mobile elements extracted from supra-subduction dykes temporarily enriched the circulating fluids and generated tremolite, antigorite, Mg-chlorite and magnetite instead of antigorite and magnetite solely. Tremolite crystallization probably ceased due to the exhaustion of Ca-rich fluids in a globally cooling fore-arc environment.
Philippe Davy - One of the best experts on this subject based on the ideXlab platform.
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Horizontal pre-asymptotic solute transport in a model Fracture with Significant density contrasts
Journal of Contaminant Hydrology, 2011Co-Authors: Jeremy Bouquain, Yves Méheust, Philippe DavyAbstract:We investigate the dispersion of a finite amount of solute after it has been injected into the laminar flow occurring in a horizontal smooth Fracture of constant aperture. When solute buoyancy is negligible, the dispersion process eventually leads to the well-known asymptotic TaylorAris dispersion regime, in which the solute progresses along the Fracture at the average fluid velocity, according to a one-dimensional longitudinal advectiondispersion process. This paper addresses more realistic configurations for which the solute-induced density contrasts within the fluid play an important role on solute transport, in particular at small and moderate times. Flow and transport are coupled, since the solute distribution impacts the variations in time of the advecting velocity field. Transport is simulated using (i) a mathematical description based on the Boussinesq approximation and (ii) a numerical scheme based on a finite element analysis. This enables complete characterization of the process, in particular at moderate times for which existing analytical models are not valid. At very short times as well as very long times, the overall downward advective solute mass flow is observed to scale as the square of the injected concentration. The asymptotic TaylorAris effective dispersion coefficient is reached eventually, but vertical density currents, which are significant at short and moderate times, are responsible for a systematic retardation of the asymptotic mean solute position with respect to the frame moving at the mean fluid velocity, as well as for a time shift in the establishment of the asymptotic dispersion regime. These delays are characterized as functions of the Péclet number and another non-dimensional number which we call advective Archimedes number, and which quantifies the ratio of buoyancy to viscous forces. Depending on the Péclet number, the asymptotic dispersion is measured to be either larger or smaller than what it would be in the absence of buoyancy effects. Breakthrough curves measured at distances larger than the typical distance needed to reach the asymptotic dispersion regime are impacted accordingly. These findings suggest that, under certain conditions, density/buoyancy effects may have to be taken into consideration when interpreting field measurement of solute transport in Fractured media. They also allow an estimate of the conditions under which density effects related to Fracture Wall roughness are likely to be significant.
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Influence of density contrasts on the solute transport through a horizontal Fracture
2010Co-Authors: Jeremy Bouquain, Yves Méheust, Philippe DavyAbstract:Contaminant transport in heterogeneous Fractured aquifers occurs mostly through the networks of intersecting Fractures. Solute transport through individual Fractures is often studied considering a continuous inflow of solute. Here we investigate the spreading of a finite amount of solute entering a Fracture of constant aperture and with no significant Wall roughness. When solute buoyancy is negligible, the dispersion process eventually leads to the well-known asymptotic Taylor-Aris[1] dispersion regime, in which the solute progresses along the Fracture at the average fluid velocity, according to a one-dimensional longitudinal advection-dispersion process. We address more realistic configurations for which the solute-induced density contrasts within the fluid play a role on solute transport, in particular at small and moderate times. Flow and transport are simulated using a mathematical description based on the Boussinesq approximation and a numerical scheme based on a finite element analysis. This enables complete characterization of the process, in particular at moderate times for which existing analytical models are not valid. Dispersion is characterized both in terms of longitudinal spreading and by computing the time evolution of the dilution index. The asymptotic Taylor-Aris effective dispersion coefficient is reached eventually, but vertical density currents, which are significant at short and moderate times, are responsible for a systematic retardation of the asymptotic mean solute position with respect to the frame moving at the mean fluid velocity, as well as for a time shift in the establishment of the asymptotic dispersion regime [2]. These delays are characterized as functions of non-dimensional numbers. In accordance with a long-existing prediction and depending on the Péclet number, the asymptotic spreading is measured to be either larger or smaller than what it would be in the absence of buoyancy effects [2]. Breakthrough curves, measured at distances larger than the typical distance needed to reach the asymptotic dispersion regime, are impacted accordingly. Additionally, as density effects affect the distribution of the solute in the Fracture thickness, the solute transfer between the Fracture and the rock matrix is impacted. Fracture-matrix transfer predictions are often based on a “perfect transverse mixing” assumption. We study the effect of the vertically-heterogeneous solute distribution and of density contrasts on this exchange, by coupling the description of transport in the Fracture and that of the transverse solute diffusion in the matrix. Our findings suggest that, under certain conditions, density/buoyancy effects may have to be taken into consideration when interpreting field measurements of solute transport in Fractured media, even in the absence of significant Fracture Wall roughness. [1] Taylor, G.I. , 1953. Dispersion of soluble matter in solvent flowing slowly through a tube. Proc. R. Soc. Lond. A219(1137), 186-203 [2] Bouquain, J., Méheust, Y., Davy, P., Horizontal pre-asymptotic solute transport in a plane Fracture with significant density contrasts, Journal of Contaminant Hydrology (2010), doi: 10.1016/j.jconhyd.2010.08.002
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Solute transport in a plane horizontal Fracture: influence of density contrasts and Fracture-matrix exchange
2010Co-Authors: Yves Méheust, Jeremy Bouquain, Laure Michel, Jean De Bremond D'ars, Philippe DavyAbstract:Fractures are preferential paths for fluids and solutes inside hard rock formations in the Earth's upper crust. We address the advective and dispersive transport of a buoyant solute in a horizontal Fracture with no Wall roughness, under laminar flow conditions. Our reference configuration is that with impervious Fracture Walls an no density-driven coupling between flow and transport; it gives rise to the classic one-dimensional longitudinal Tayor-Aris dispersion process. In reality the solute usually has a negative buoyancy, so that the fluid density is spatially distributed according to the solute concentration field, which induces significant perturbations to the Poiseuille flow inside the Fracture. We study this impact of density constrasts on the longitudinal dispersion, using a two-dimensional finite elements numerical simulation. The asymptotic TaylorAris effective dispersion coefficient is observed to be reached eventually, but buoyancy effects at short and moderate times are responsible for a systematic retardation of the asymptotic mean solute position with respect to the frame moving at the mean fluid velocity, as well as for a time shift in the establishment of the asymptotic dispersion regime. We characterize these time delays as a function of the Péclet number and of another non-dimensional number that quantifies the ratio of buoyancy to viscous forces. Depending on the Péclet number, the asymptotic dispersion is measured to be either larger or smaller than what it would be in the absence of buoyancy effects. Breakthrough curves (an important measurement in hydrogeological applications) measured at distances larger than the typical distance needed to reach the asymptotic dispersion regime are impacted accordingly. We also discuss conditions under which density effects related to Fracture Wall roughness are likely to be significant, or not. Another effect that can strongly influence the transport process is the small but finite porosity of the rock matrix, which allows part of the solute present in the vicinity of the Fracture Wall do diffuse into the matrix. We carry an experimental study of this effect. The analog Fracture model consists of a 1000x50x5 mm3 plexiglass box with a porous lower Wall made of 1mm-large glass beads. A permanent laminar water flow is forced through the Fracture at controlled mean velocity (~ 1mm/s). A dye (patent blue) injection system simulates a point source of contaminant along the center plane of the experimental Fracture. The two-dimensional equivalent longitudinal concentration field is measured as a function of time using a visualization system based on 4 cameras positioned side by side.Mass transfer between the Fracture and the bounding porous matrix is measured at different volumetric flows and for various concentrations of the injected dye, and this in different geometries (roughness) of the Fracture-matrix interface. Here also, buoyancy effects play a significant role in the trapping of the solute in the vicinity of the porous Wall
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Solute transport in a plane horizontal Fracture: influence of density contrasts and Fracture-matrix exchange
2010Co-Authors: Y. Meheust, Jeremy Bouquain, Laure Michel, Jean De Bremond D'ars, Philippe DavyAbstract:Fractures are preferential paths for fluids and solutes inside hard rock formations in the Earth's upper crust. We address the advective and dispersive transport of a buoyant solute in a horizontal Fracture with no Wall roughness, under laminar flow conditions. Our reference configuration is that with impervious Fracture Walls an no density-driven coupling between flow and transport; it gives rise to the classic one-dimensional longitudinal Tayor-Aris dispersion process. In reality the solute usually has a negative buoyancy, so that the fluid density is spatially distributed according to the solute concentration field, which induces significant perturbations to the Poiseuille flow inside the Fracture. We study this impact of density constrasts on the longitudinal dispersion, using a two-dimensional finite elements numerical simulation. The asymptotic TaylorAris effective dispersion coefficient is observed to be reached eventually, but buoyancy effects at short and moderate times are responsible for a systematic retardation of the asymptotic mean solute position with respect to the frame moving at the mean fluid velocity, as well as for a time shift in the establishment of the asymptotic dispersion regime. We characterize these time delays as a function of the Peclet number and of another non-dimensional number that quantifies the ratio of buoyancy to viscous forces. Depending on the Peclet number, the asymptotic dispersion is measured to be either larger or smaller than what it would be in the absence of buoyancy effects. Breakthrough curves (an important measurement in hydrogeological applications) measured at distances larger than the typical distance needed to reach the asymptotic dispersion regime are impacted accordingly. We also discuss conditions under which density effects related to Fracture Wall roughness are likely to be significant, or not. Another effect that can strongly influence the transport process is the small but finite porosity of the rock matrix, which allows part of the solute present in the vicinity of the Fracture Wall do diffuse into the matrix. We carry an experimental study of this effect. The analog Fracture model consists of a 1000x50x5 mm3 plexiglass box with a porous lower Wall made of 1mm-large glass beads. A permanent laminar water flow is forced through the Fracture at controlled mean velocity (~ 1mm/s). A dye (patent blue) injection system simulates a point source of contaminant along the center plane of the experimental Fracture. The two-dimensional equivalent longitudinal concentration field is measured as a function of time using a visualization system based on 4 cameras positioned side by side.Mass transfer between the Fracture and the bounding porous matrix is measured at different volumetric flows and for various concentrations of the injected dye, and this in different geometries (roughness) of the Fracture-matrix interface. Here also, buoyancy effects play a significant role in the trapping of the solute in the vicinity of the porous Wall
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Solute transport through a Fracture with significant density effects
2010Co-Authors: Jeremy Bouquain, Yves Méheust, Philippe DavyAbstract:Contaminant transport in heterogeneous Fractured aquifers occurs mostly through the networks of intersecting Fractures. Solute transport through individual Fractures is often studied considering a continuous inflow of solute. Here we investigate the spreading of a finite amount of solute entering a Fracture of constant aperture and with no significant Wall roughness. When solute buoyancy is negligible, the dispersion process eventually leads to the well-known asymptotic Taylor-Aris dispersion regime, in which the solute progresses along the Fracture at the average fluid velocity, according to a one-dimensional longitudinal advection-dispersion process. We address more realistic configurations for which the solute-induced density contrasts within the fluid play a role on solute transport, in particular at small and moderate times. Flow and transport are simulated using a mathematical description based on the Boussinesq approximation and a numerical scheme based on a finite element analysis. This enables complete characterization of the process, in particular at moderate times for which existing analytical models are not valid. The asymptotic Taylor-Aris effective dispersivity is reached eventually, but secondary vertical density currents, which are significant at short and moderate times, are responsible for a systematic retardation of the asymptotic mean solute position with respect to the frame moving at the mean fluid velocity, as well as for a time shift in the establishment of the asymptotic dispersion regime. These delays are characterized as functions of non-dimensional numbers. In accordance with a long-existing prediction and depending on the Péclet number, the asymptotic spreading is measured to be either larger or smaller than what it would be in the absence of buoyancy effects. Breakthrough curves measured at distances larger than the typical distance needed to reach the asymptotic dispersion regime are impacted accordingly. These findings suggest that, under certain conditions, density/buoyancy effects may have to be taken into consideration when interpreting field measurements of solute transport in Fractured media, even in the absence of significant Fracture Wall roughness.
