Effects of high variance of fracture transmissivity on transport and sorption at different scales in a discrete model for fractured rocks

Abstract A three-dimensional (3-D) variable-aperture fracture network model for flow and transport in fractured crystalline rocks has been applied to study the effects of large variability in fracture transmissivity on non-sorbing and sorbing tracer transport, and scale effects in transport distance. The variable-aperture character of the fractures is introduced into a 3-D network model through a library of single-fracture permeabilities and associated particle transport residence time spectra. Sorption onto the fracture walls is added by a mathematical model for linear sorption. The resulting variable-aperture fracture network model, VAPFRAC, can handle flow and transport from single-fracture scale to the multiple-fracture scale. The model produces multi-peak transport breakthrough curves even for relatively moderate values of the fracture transmissivity variance. These breakthrough curves display dispersion on two different scales in the same way as has been observed in several field experiments conducted in crystalline rocks. The multi-peak structure is due to so-called channeling. For high values of the fracture transmissivity variance the solute transport is unevenly distributed and the channeling effects are more prominent. The effect of linear sorption is not just a simple translation in mean residence time as in a homogeneous medium. The dispersion characteristics of the breakthrough curves also change when linear sorption is included. The degree of the change depends strongly on the fracture transmissivity variance, as does the translation. In particular, with a high fracture transmissivity variance the translation in mean residence time due to sorption is significantly smaller compared to the cases with a low fracture transmissivity variance. Finally, the high variability in the model output data suggests that extrapolation of results from a particular tracer experiment will be highly uncertain.

[1]  Björn Dverstorp,et al.  Application of the discrete fracture network concept with field data: Possibilities of model calibration and validation , 1989 .

[2]  Björn Dverstorp,et al.  Discrete fracture network interpretation of field tracer migration in sparsely fractured rock , 1992 .

[3]  Chin-Fu Tsang,et al.  Multiple-Peak Response to Tracer Injection Tests in Single Fractures: A Numerical Study , 1991 .

[4]  O. Stephansson,et al.  Experimental Technique For Aperture Studies of Intersecting Joints , 1993 .

[5]  Ivars Neretnieks,et al.  A note on fracture flow dispersion mechanisms in the ground , 1983 .

[6]  R. Gillham,et al.  Laboratory and field measurements of non-equilibrium transport in the Borden aquifer, Ontario, Canada , 1992 .

[7]  S. P. Neuman.  Universal scaling of hydraulic conductivities and dispersivities in geologic media , 1990 .

[8]  I. Neretnieks,et al.  Tracer movement in a single fissure in granitic rock: Some experimental results and their interpretation , 1982 .

[9]  Björn Dverstorp,et al.  Conditional simulations of fluid flow in three-dimensional networks of discrete fractures , 1987 .

[10]  Ivars Neretnieks,et al.  Flow and nuclide transport in fractured media: the importance of the flow-wetted surface for radionuclide migration , 1993 .

[11]  Chin-Fu Tsang,et al.  A variable aperture fracture network model for flow and transport in fractured rocks , 1992 .

[12]  L. Smith,et al.  Retardation of sorbing solutes in fractured media , 1994 .

[13]  O. Levenspiel.  Chemical Reaction Engineering , 1972 .

[14]  R. L. Naff,et al.  Nonreactive and reactive solute transport in three-dimensional heterogeneous porous media: Mean displacement, plume spreading, and uncertainty , 1994 .

[15]  N. Barton,et al.  Aperture measurements and flow experiments using transparent replicas of rock joints , 1991 .

[16]  Chin-Fu Tsang,et al.  Channel model of flow through fractured media , 1987 .

[17]  Paul A. Witherspoon,et al.  A Model for Steady Fluid Flow in Random Three‐Dimensional Networks of Disc‐Shaped Fractures , 1985 .

[18]  G. Marsily,et al.  Modeling fracture flow with a stochastic discrete fracture network: calibration and validation: 1. The flow model , 1990 .

[19]  Anders Rasmuson,et al.  Radionuclide Transport in Fast Channels in Crystalline Rock , 1986 .

[20]  Chin-Fu Tsang,et al.  Flow and tracer transport in fractured media: A variable aperture channel model and its properties , 1988 .

[21]  Ivars Neretnieks,et al.  Fluid flow and solute transport in a network of channels , 1993 .

[22]  K. Raven,et al.  Interpretation of field tracer tests of a single fracture using a transient solute storage model , 1988 .

[23]  I. Neretnieks,et al.  Analysis of Some Laboratory Tracer Runs in Natural Fissures , 1985 .

[24]  J. Bear.  Dynamics of Fluids in Porous Media , 1975 .

[25]  TRACER TRANSPORT IN FRACTURES : ANALYSIS OF FIELD DATA BASED ON A VARIABLE-APERTURE CHANNEL MODEL , 1991 .

[26]  Emmanuel Ledoux,et al.  Modeling fracture flow with a stochastic discrete fracture network: Calibration and validation: 2. The transport model , 1990 .

[27]  Thomas D. Waite,et al.  Transport of chromium and selenium in the suboxic zone of a shallow aquifer: Influence of redox and adsorption reactions , 1994 .

[28]  K. Raven,et al.  A Field Example of Measuring Hydrodynamic Dispersion in a Single Fracture , 1985 .

[29]  Chin-Fu Tsang,et al.  Flow and tracer transport in a single fracture: A stochastic model and its relation to some field observations , 1988 .

[30]  K. Mardia.  Statistics of Directional Data , 1972 .

[31]  J. Gale,et al.  Comparison of coupled fracture deformation and fluid flow models with direct measurements of fracture pore structure and stress-flow properties , 1987 .