NUMERICAL INVESTIGATIONS OF PASSIVE AND REACTIVE FLOW THROUGH GENERIC SINGLE FRACTURES WITH HETEROGENEOUS PERMEABILITY

Abstract

Numerical investigations of the relationship between permeability distributions representing fracture apertures and the transport of inert and reactive solutes have been carried out. To model reactive transport, a lattice gas method for fluid flow was coupled to a finite-difference solution to the advection-dispersion equation with the inclusion of mineral dissolution and precipitation reactions. This enabled us to model reactive flow in heterogeneous media with a feedback between geochemical alteration and the flow regime. We modelled the transport of both inert tracers and reactive solutes through several realisations of different permeability distributions. The numerically simulated distributions were generated to represent transport through a fracture of 150 mx50 m area. The mean of a contaminant plume moved sublinearly through the distribution with the mean velocity decreasing through time. The standard deviation of a contaminant plume throughout the medium scales as σ~tβ. We have found that β ranges from 0.5 to 0.9 for different distributions, for Gaussian transport β=0.5. The introduction of reactive solutes changes the flow and transport properties. Quartz dissolution, for example, increases the permeability and creates channels which focus the flow and increase the dissolution to create a positive feedback until the solution reaches saturation. The roughness of the dissolution pattern is dependent on the initial geometry and the Damkohler number.