Abstract
Two-dimensional percolation networks have been used to model a disordered and fractured porous medium. The advantage of percolation networks is that they allow the flow and transport properties of the system to be systematically studied as a function of the connectivity of the fractures and/or the permeable regions. The aim of this research is to study hydrodynamic dispersion in such networks, and to investigate the behavior of the longitudinal dispersion coefficient DL with binary and log-normally distributed hydraulic conductivity fields. In particular, the study focuses on the behavior of DL at the percolation threshold pc, where the insufficiency of flow field homogenization and the limited number of tortuous paths for flow and transport force DL to behave anomalously, i.e., to be scale- and time-dependent. The simulations indicate that the DL population taken over a large number of the network realizations resembles a log-normal distribution, hence indicating that, unlike the hydraulic conductivity, DL is not a self-averaged property whose variance should tend to zero when the size of the system tends to infinity. In addition, it was found that the power law that characterizes the scale dependence of DL is contingent upon its computation method. Moreover, DL is found to have a completely different behavior in networks with low and high connectivities.
Résumé
Des réseaux de percolation bi-dimensionnels ont été utilisés pour modéliser des milieux poreux fracturés et désordonnés. L’avantage de ces réseaux est qu’ils permettent d’étudier les propriétés d’écoulement et de transport en fonction de la connexité des fractures et/ou des zones perméables. L’objectif est d’appréhender la dispersion hydrodynamique et d’investiguer le comportement du coefficient de dispersion longitudinal DL pour des réseaux ayant une distribution des conductivités hydrauliques binaire et log-normale. En particulier, le comportement de DL a été étudié au seuil de percolation pc, là où l’inhomogénéité des vitesses du fluide et le nombre limité de chemins tortueux disponibles pour l’écoulement et le transport entraînent un comportement anormal de DL, à savoir, une dépendance vis-à-vis des échelles de temps et d’espace. Les simulations montrent que les populations de DL définies pour un grand nombre de réseaux ressemblent à des distributions log-normales, indiquant que, contrairement à la conductivité hydraulique, DL n’est pas une propriété dont la variance tend vers zéro lorsque la taille du système tend vers l’infini. Il a également été trouvé que les lois de puissance qui caractérisent la dépendance d’échelle de DL découlent directement de la méthode de calcul. Enfin, les simulations engendrent un comportement très différent de DL dans des réseaux faiblement ou fortement connectés.
Resumen
Simulaciones de transporte de solutos en medio heterogéneo utilizando redes de precolación 2D con campos de conductividad no correlacionados. Se han utilizado redes de percolación en dos dimensiones para modelizar un medio poroso fracturado y desordenado. La ventaja de redes de percolación es que permiten estudiar sistemáticamente las propiedades de flujo y transporte del sistema en función de la conectividad de las fracturas y/o regiones permeables. El objetivo de esta investigación es estudiar la dispersión hidrodinámica en las redes mencionadas, e investigar el comportamiento del coeficiente de dispersión longitudinal DL con campos de conductividad hidráulica que tienen distribución log-normal y binaria. El estudio se enfoca particularmente en el comportamiento de DL cuando se alcanza el threshold de percolación pc, donde la insuficiencia de homogenización del campo de flujo y el número limitado de trayectorias tortuosas de flujo y fuerza de transporte DL se comportan de manera anómala, i.e., son dependientes del tiempo y la escala. Las simulaciones indican que la población DL tomada en un número grande en las redes realizadas se asemeja a una distribución log-normal, indicando por lo tanto que, a diferencia de la conductividad hidráulica, DL no es una propiedad auto-promediable cuya variación tienda a cero cuando el tamaño del sistema tiende al infinito. Se encontró además que la ley potencia que caracteriza la dependencia en escala de DL es dependiente del método de cálculo. Más aún, se encontró que DL tiene un comportamiento completamente distinto en las redes de alta y baja conectividad.
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Acknowledgements
The authors would like to thank the Natural Sciences and Engineering Research Council of Canada and the Formation des Chercheurs et Aide à la Recherche of Québec for their financial support. They would also like to deeply thank Dr. Muhammad Sahimi for his help and support.
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Notation List
- d
-
Dimensionality or Euclidean dimension
- dB
-
Backbone fractal dimension, dB=d-βB/ν
- df
-
Sample-spanning cluster fractal dimension, df=d-β/ν
- DL
-
Longitudinal dispersion coefficient
- e
-
Exponent of Ke
- i
-
Hydraulic gradient
- K
-
Hydraulic conductivity
- Ke
-
Equivalent hydraulic conductivity
- l
-
Bond length
- L
-
Network length
- n
-
Number of bonds in one direction
- ne
-
Effective porosity
- p
-
Probability or fraction of open bonds
- pc
-
Percolation threshold
- Pe
-
Peclet number, Pe=u*ξp/Dm where Dm is the molecular diffusion coefficient
- Q
-
Flow rate
- R2
-
Determination coefficient
- S
-
Surface
- t
-
Travel tim
- v
-
Velocity, \( \hat{v} = {\text{K*i}} \)
- x
-
Position along the longitudinal axis
- XA
-
Fraction of open bonds belonging to the sample-spanning cluster
- XB
-
Fraction of open bonds belonging to the backbone
- αL
-
Longitudinal dispersivity
- β
-
Exponent of XA
- βB
-
Exponent of XB
- χ
-
Exponent of the ~Lχ type law
- δ
-
Exponent of the ~tδ type law
- ν
-
Exponent of the percolation correlation length (ξp)
- θ
-
(e - β)/ν
- θB
-
(e - βB)/ν
- σt
-
Standard deviation of the particle (mass) distributions in time at a given location
- σx
-
Standard deviation of the particle (mass) distributions in space at a given time
- ξp
-
Percolation correlation length
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Rivard, C., Delay, F. Simulations of solute transport in fractured porous media using 2D percolation networks with uncorrelated hydraulic conductivity fields. Hydrogeology Journal 12, 613–627 (2004). https://doi.org/10.1007/s10040-004-0363-z
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DOI: https://doi.org/10.1007/s10040-004-0363-z