Research paper
Contaminant transport in fractured rocks with significant matrix permeability, using natural fracture geometries

https://doi.org/10.1016/S0169-7722(96)00096-4Get rights and content

Abstract

Some results from numerical models of flow and contaminant transport in fractured permeable rocks, where fractures are more conductive than rock matrix, are described. The 2D flow field in the fractured and permeable rock matrix is calculated using a finite difference, ‘conductance mesh’ method, and the contaminant transport is simulated by particle tracking methods using an advection-biased, random walk technique. The model is applied to simulated and naturally occurring fracture patterns. The simulated pattern is an en echelon array of unconnected fractures, as an example of a common, naturally occurring fracture geometry. Two natural fracture patterns are used: one of unconnected, sub-parallel fractures and one with oblique fracture sets which is well connected. Commonly occurring matrix permeability and fracture aperture values are chosen. The simulations show that the presence of fractures creates complex and heterogeneous flow fields and contaminant distribution in the permeable rock matrix. The modelling results have shown that some effects are non-intuitive and therefore difficult to foresee without the help of a model. With respect to contaminant transport rates and plume heterogeneity, it was found that fracture connectivity (crucial when the matrix is impermeable) can play a secondary role to fracture orientation and density. Connected fracture systems can produce smooth break-through curves of contaminants summed over, for example, a bore-hole length, whereas in detail the contaminant plume is spatially highly heterogeneous. Close to a constant-pressure boundary (e.g. an extraction bore-hole), flow and contaminants can be channelled by fractures. Thus observations at a bore-hole may suggest that contaminants are largely confined to the fracture system, when, in fact, significant contamination resides in the matrix.

References (33)

  • K.G. Raven et al.

    Water flow in a natural rock fracture as a function of stress and sample size

    Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

    (1985)
  • D.T. Snow

    Frequency and apertures of joints in rocks

    Int. J. Rock Mech.

    (1970)
  • S.C. Bandis et al.

    Predicted and measured hydraulic conductivity of rock joints

  • B. Berkowitz

    Analysis of fracture connectivity using percolation theory

    Math. Geol.

    (1995)
  • B. Berkowitz et al.

    Mass transfer at fracture intersections: an evaluation of mixing models

    Water Resour. Res.

    (1994)
  • M.A. Cacas et al.

    Modelling fracture flow with a stochastic discrete fracture network: calibration and validation. 2. The transport model

    Water Resour. Res.

    (1990)
  • J. Chilés et al.

    3D stochastic simulation of fracture network and flow at Stripa conditioned on observed fractures and calibrated on measured flow rates

  • G. Dagan

    Flow and Transport in Porous Formations

  • G. de Marsily

    Spatial variability of properties in porous media; a stochastic approach

  • C.W. Fetter

    Applied Hydrogeology

  • L.W. Gelhar

    Stochastic analysis of flow in heterogeneous porous media

  • E. Hakami et al.

    Aperture measurements and flow experiments using transparent replicas

  • A. Herbert et al.

    A prediction of flows to be measured in the Stripa D-hole experiment: an application of the fracture network approach

  • J.C.S. Long et al.

    The relationship of the degree of interconnection to permeability in fracture networks

    J. Geophys. Res.

    (1985)
  • L. Luckner et al.

    Migration Processes in the Soil and Groundwater Zone

  • L. Moreno et al.

    Flow and tracer transport in a single fracture: a stochastic model and its relation to some field observations

    Water Resour. Res.

    (1988)
  • Cited by (53)

    • Dual length scale non-local model to represent damage and transport in porous media

      2021, Computer Methods in Applied Mechanics and Engineering
    View all citing articles on Scopus
    1

    Present address: Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK.

    View full text