Hydrogeology of low permeability media http://www.clay-permeability.net Research overview Mon, 28 Dec 2009 12:38:26 +0000 http://wordpress.org/?v=2.9.1 en hourly 1 Project overview: A geostatistical methodology for modelling groundwater flow and transport in low-permeability media – application on Boom Clay, Ieper Clay and Toarcian argillites http://www.clay-permeability.net/2009/12/a-geostatistical-methodology-for-modelling-groundwater-flow-and-transport-in-low-permeability-media-%e2%80%93-application-on-boom-clay-ieper-clay-and-toarcian-argillites/ http://www.clay-permeability.net/2009/12/a-geostatistical-methodology-for-modelling-groundwater-flow-and-transport-in-low-permeability-media-%e2%80%93-application-on-boom-clay-ieper-clay-and-toarcian-argillites/#comments Fri, 25 Dec 2009 10:25:40 +0000 Marijke Huysmans http://www.clay-permeability.net/?p=19 Fluid flow in permeable media has been extensively studied and analyzed in several contexts, especially in the development of groundwater resources and the recovery of petroleum. By comparison, less effort has been spent to groundwater flow and transport of pollutants in low-permeability media. Recently low-permeability layers receive more attention because their low hydraulic conductivities are interesting for several important applications: landfilling of industrial and municipal waste, storage of carbon dioxide, oil and gas reservoir formation and storage of high-level nuclear waste. In all these applications, it is of great importance that the behaviour of groundwater and pollutants is well understood and can be predicted. This is unfortunately not always the case. Too often, phenomena that can not be explained with classical models are observed in low-permeability media: anomalous groundwater pressures, regional permeability values much larger than expected from laboratory tests, concentration measurements that deviate from the predicted values, etc. These problems are often due to (1) inappropriate application of methods originally developed for high-permeability media, (2) the limited data availability about the main transport parameters in low permeability media caused by experimental difficulties, (3) not taking into account the heterogeneity of the different transport parameters and (4) not taking into account of fractures.

In this work, a rigorous methodology was developed for modelling flow and transport in low-permeability media. The development and application of the methodology has resulted in general recommendations for modelling flow and transport in low-permeability for various applications: (1) the relative importance of advection and diffusion should be determined using a Péclet number criterion suitable for low-permeability environments; (2) secondary information such as grain size, lithology and geophysical parameters are very useful to complement and compensate the often limited knowledge of the transport parameters; (3) heterogeneity of hydraulic conductivity should be incorporated; (4) heterogeneity of the diffusion parameters, i.e., the diffusion coefficient and diffusion accessible porosity, should be incorporated and (5) fractures, if present, should be incorporated in flow and transport models of low-permeability media.

Application of the method on potential radionuclide transport in the Boom Clay (Belgium) shows that the effect of K heterogeneity and fractures on flow and transport is limited and that is does not affect the main safety function of the Boom Clay. The heterogeneity of the diffusion parameters has a larger effect on the radionuclide fluxes but neglecting the heterogeneity of the diffusion parameters is a safe conservative assumption since the radionuclide fluxes calculated with heterogeneous models are smaller than the fluxes calculated with a homogeneous model.

Application of the method on the Ieper Clay (Belgium) shows that the heterogeneity of hydraulic conductivity and the diffusion parameters has a large effect on the calculated radionuclide fluxes. Future flow and transport models of the Ieper Clay should thus incorporate the heterogeneity of hydraulic conductivity, the diffusion coefficient and the diffusion accessible porosity.

Application of the method on modeling of chloride transport in the Toarcian argillites (France) shows that incorporating the heterogeneity of the diffusion parameters results in a better correspondence between measured and calculated chloride concentration values.

 

Selected publications

]]> http://www.clay-permeability.net/2009/12/a-geostatistical-methodology-for-modelling-groundwater-flow-and-transport-in-low-permeability-media-%e2%80%93-application-on-boom-clay-ieper-clay-and-toarcian-argillites/feed/ 0 Paper: Equivalent diffusion coefficient and equivalent diffusion accessible porosity http://www.clay-permeability.net/2009/12/equivalent-diffusion-coefficient-and-equivalent-diffusion/ http://www.clay-permeability.net/2009/12/equivalent-diffusion-coefficient-and-equivalent-diffusion/#comments Fri, 25 Dec 2009 10:24:26 +0000 Marijke Huysmans http://www.clay-permeability.net/?p=17 Abstract
Diffusion is an important transport process in low permeability media, which play an important role in contamination and remediation of natural environments. The calculation of equivalent diffusion parameters has however not been extensively explored. In this paper, expressions of the equivalent diffusion coefficient and the equivalent diffusion accessible porosity normal to the layering in a layered porous medium are derived based on analytical solutions of the diffusion equation. The expressions show that the equivalent diffusion coefficient changes with time. It is equal to the power average with p = -0.5 for small times and converges to the harmonic average for large times. The equivalent diffusion accessible porosity is the harmonic average of the porosities of the individual layers for all times. The expressions are verified numerically for several test cases.

Huysmans M. and Dassargues A., 2007, Equivalent diffusion coefficient and equivalent diffusion accessible porosity of a stratified porous medium, Transport in Porous Media, 66(3), 421-438

Link to article:
http://www.springerlink.com/content/67u614v13108536p/

]]> http://www.clay-permeability.net/2009/12/equivalent-diffusion-coefficient-and-equivalent-diffusion/feed/ 0 Paper: Stochastic analysis of the effect of spatial variability of diffusion parameters on radionuclide transport in a low permeability clay layer http://www.clay-permeability.net/2009/12/stochastic-analysis-of-the-effect-of-spatial-variability-of-diffusion-parameters-on-radionuclide-transport-in-a-low-permeability-clay-layer/ http://www.clay-permeability.net/2009/12/stochastic-analysis-of-the-effect-of-spatial-variability-of-diffusion-parameters-on-radionuclide-transport-in-a-low-permeability-clay-layer/#comments Fri, 25 Dec 2009 10:23:35 +0000 Marijke Huysmans http://www.clay-permeability.net/?p=15 Abstract
Most studies that incorporate subsurface heterogeneity in groundwater flow and transport models only analyze and simulate the spatial variability of hydraulic conductivity. Heterogeneity of the other flow and transport parameters is usually neglected. This approach is often justified, but there are, however, cases in which disregarding the heterogeneity of the other flow and transport parameters can be questionable. In low permeability media, for instance, diffusion is often the dominant transport mechanism. It therefore seems logical to incorporate the spatial variability of the diffusion parameters in the transport model. This study therefore analyses and simulates the spatial variability of the effective diffusion coefficient and the diffusion accessible porosity with geostatistical techniques and incorporates their heterogeneity in the transport model of a low permeability formation. The formation studied was Boom clay (Belgium), a candidate host rock for the deep geological disposal of high-level radioactive waste. The calculated output radionuclide fluxes of this model are compared with the fluxes calculated with a homogeneous model and a model with a heterogeneous hydraulic conductivity distribution. This analysis shows that the heterogeneity of the diffusion parameters has a much larger effect on the calculated output radionuclide fluxes than the heterogeneity of hydraulic conductivity in the low permeability medium under study.

Huysmans M. and Dassargues A., 2006, Stochastic analysis of the effect of spatial variability of diffusion parameters on radionuclide transport in a low permeability clay layer, Hydrogeology Journal, 14(7), 1094-1106

Link to article:
http://www.springerlink.com/content/x032w11v68550634/

]]> http://www.clay-permeability.net/2009/12/stochastic-analysis-of-the-effect-of-spatial-variability-of-diffusion-parameters-on-radionuclide-transport-in-a-low-permeability-clay-layer/feed/ 0 Paper: Review of the use of Péclet numbers to determine the relative importance of advection and diffusion in low permeability environments http://www.clay-permeability.net/2009/12/review-of-the-use-of-peclet-numbers-to-determine-the-relative-importance-of-advection-and-diffusion-in-low-permeability-environments/ http://www.clay-permeability.net/2009/12/review-of-the-use-of-peclet-numbers-to-determine-the-relative-importance-of-advection-and-diffusion-in-low-permeability-environments/#comments Fri, 25 Dec 2009 10:22:50 +0000 Marijke Huysmans http://www.clay-permeability.net/?p=13 Abstract
In low permeability environments, transport by advection is often neglected based on a Péclet number criterion. Such a criterion usually states that if the Péclet number (Pe) is much smaller than 1, diffusion dominates over advection and transport may be modeled considering diffusion only. Unfortunately, up to 10 different Péclet number definitions exist and for a particular case these different definitions lead to very diverse Péclet number values, differing several orders of magnitude from each other. In this paper, the different Péclet number definitions are therefore evaluated on their ability to determine the relative importance of transport by advection and by diffusion in low permeability environments. This is done by comparing the results of the analytical solution for pure diffusion with the analytical solution for diffusion, advection and dispersion for a large number of different input parameter values. The relation between the different Péclet numbers and the difference between the calculated concentration considering diffusion only and the calculated concentration considering both diffusion and advection is studied. These calculations show that some Péclet number definitions are not well suited to decide whether advection may be neglected in low permeability media.

Huysmans M. and Dassargues A., 2005, Review of the use of Péclet numbers to determine the relative importance of advection and diffusion in low permeability environments, Hydrogeology Journal, 13(5-6), 895-904

Link to article:
http://www.springerlink.com/content/w18vmjv839f7p32y/

]]> http://www.clay-permeability.net/2009/12/review-of-the-use-of-peclet-numbers-to-determine-the-relative-importance-of-advection-and-diffusion-in-low-permeability-environments/feed/ 0 Paper: Hydrogeological modeling of radionuclide transport in low permeability media: a comparison between Boom Clay and Ypresian Clay http://www.clay-permeability.net/2009/12/hydrogeological-modeling-of-radionuclide-transport-in-low-permeability-media-a-comparison-between-boom-clay-and-ypresian-clay/ http://www.clay-permeability.net/2009/12/hydrogeological-modeling-of-radionuclide-transport-in-low-permeability-media-a-comparison-between-boom-clay-and-ypresian-clay/#comments Fri, 25 Dec 2009 10:21:18 +0000 Marijke Huysmans http://www.clay-permeability.net/?p=9 Abstract
Deep low-permeability clay layers are considered as suitable environments for disposal of high-level radioactive waste. In Belgium, the Boom Clay is the reference host formation and the Ypresian Clay an alternative host formation for research and safety and feasibility assessment of deep disposal of nuclear waste. In this study, two hydrogeological models are built to calculate the radionuclide fluxes that would migrate from a potential repository through these two clay formations. Transport parameter heterogeneity is incorporated in the models using geostatistical co-simulations of hydraulic conductivity, diffusion coefficient and diffusion accessible porosity. The calculated radionuclide fluxes in the two clay formations are compared. The results show that in the Ypresian Clay larger differences between the fluxes through the lower and the upper clay boundary occur, larger total output radionuclide amounts are calculated and a larger effect of parameter heterogeneity on the calculated fluxes is observed, compared to the Boom Clay.

Huysmans M. and Dassargues A., 2006, Hydrogeological modeling of radionuclide transport in low permeability media: a comparison between Boom Clay and Ypresian Clay, Environmental Geology, 50 (1), 122-131

Link to article:
http://www.springerlink.com/content/k3w6q86n32527137/

]]> http://www.clay-permeability.net/2009/12/hydrogeological-modeling-of-radionuclide-transport-in-low-permeability-media-a-comparison-between-boom-clay-and-ypresian-clay/feed/ 0 Paper: Stochastic analysis of the effect of heterogeneity and fractures on radionuclide transport in a low permeability clay layer http://www.clay-permeability.net/2009/12/stochastic-analysis-of-the-effect-of-heterogeneity-and-fractures-on-radionuclide-transport-in-a-low-permeability-clay-layer/ http://www.clay-permeability.net/2009/12/stochastic-analysis-of-the-effect-of-heterogeneity-and-fractures-on-radionuclide-transport-in-a-low-permeability-clay-layer/#comments Fri, 25 Dec 2009 10:19:59 +0000 Marijke Huysmans http://www.clay-permeability.net/?p=7 Abstract
Deep low-permeability clay layers are considered as safe environments for disposal of high-level radioactive waste. In Belgium, the Boom Clay is a candidate host rock for deep geological disposal. In this study, we analyze the effects of fractures and spatially variable hydraulic conductivity on radionuclide migration through the clay. Fracture geometry and properties are simulated with Monte Carlo simulation. The heterogeneity of hydraulic conductivity is simulated by direct sequential co-simulation using measurements of hydraulic conductivity and four types of secondary variables. The hydraulic conductivity and fracture simulations are used as input for a transport model. Radionuclide fluxes computed with this heterogeneous model are compared with fluxes obtained with a homogeneous model. The output fluxes of the heterogeneous model differ at most 8% from the homogeneous model. The main safety function of the Boom Clay is thus not affected by the fractures and the spatial variability of hydraulic conductivity.

Huysmans M. and Dassargues A., 2005, Stochastic analysis of the effect of heterogeneity and fractures on radionuclide transport in a low permeability clay layer, Enviromental Geology, 48(7), 920-930

Link to article:
http://www.springerlink.com/content/q2048m9886216g64/

]]> http://www.clay-permeability.net/2009/12/stochastic-analysis-of-the-effect-of-heterogeneity-and-fractures-on-radionuclide-transport-in-a-low-permeability-clay-layer/feed/ 0 Paper: Effect of excavation induced fractures on radionuclide migration through the Boom Clay (Belgium) http://www.clay-permeability.net/2009/12/effect-of-excavation-induced-fractures-on-radionuclide-migration-through-the-boom-clay-belgium/ http://www.clay-permeability.net/2009/12/effect-of-excavation-induced-fractures-on-radionuclide-migration-through-the-boom-clay-belgium/#comments Fri, 25 Dec 2009 10:18:50 +0000 Marijke Huysmans http://www.clay-permeability.net/?p=4 Abstract
In Belgium, the Boom Clay is being evaluated as a potential host formation for the disposal of high-level nuclear waste. In order to investigate this option, an underground research facility composed of two access shafts and 200mof galleries was excavated. Excavation induced fractures were observed in a zone of approximately 1 m around the galleries. In this study, the potential effect of these fractures on radionuclide migration in the Boom Clay is investigated. Therefore, a hydrogeological model of the clay is built with a radionuclide source in themiddle of the clay layer surrounded by different fracture configurations. Two types of fracture configurations are inserted in the model. The first type of fracture properties is drawn stochastically from the probability distributions of the properties of the fractures observed around previously excavated galleries. These fracture patterns are considered to be realistic although in this study it is conservatively assumed that no self-sealing occurs. The model is run for a large number of stochastically drawn fracture configurations and the results are compared to a model without fractures. These calculations show that the radionuclide fluxes through the clay are not significantly influenced by these fractures. For the second type of fracture configurations, the fracture properties are varied over a much larger range. Hypothetical fractures with much higher values of fracture extent, aperture, dip and frequency than observed are modeled. With these hypothetical fracture configurations, the critical values of the fracture parameters are determined that must be exceeded to have a significant effect on the radionuclide fluxes through the clay. These calculations show that the extent of the fractured zone has the largest effect on radionuclide migration. The other fracture parameters (aperture, spacing and dip) have a limited effect on the radionuclide fluxes. To obtain a total radionuclide flux through the lower clay boundary that is respectively 10%, 50% and 100% larger than without a fractured zone, the extent of the hypothetical fractured zone should be respectively 27.97 m, 43.86 m and 46.92 m. Such fractures are not expected to be generated by mechanical excavation in the Boom Clay. The obtained critical extent values of the hypothetical fractures are a factor 30 to 40 higher than the measured values of the extent of the excavation disturbed zone. These calculations thus indicate that it is very unlikely that the extent of the fractured zone around the galleries will be large enough to have a significant effect on the radionuclide fluxes through the Boom Clay. This conclusion is further supported by the conservative assumption that no self-sealing occurs. These calculations can function as a preliminary robustness test in ongoing safety analysis studies.

Huysmans M., Berckmans A. and Dassargues A., 2006, Effect of excavation induced fractures on radionuclide migration through the Boom Clay (Belgium), Applied Clay Science, 33(3-4), 207-218

Link to article:
linkinghub.elsevier.com/retrieve/pii/S0169131706001001

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