Effects of Fracture Geometry on Contaminant Transport

Abstract

An invasion percolation (IP) model was used to illustrate the effects of gravity on DNAPL migration into a horizontal water saturated fracture. While gravity is typically neglected in the conventional approach, this work demonstrated that gravity should often be included when modelling DNAPL invasion in water saturated fractures and provides an equation estimating the difference in invasion pattern between simulations including or neglecting gravity. The IP model was further utilized to examine the invasion of DNAPL saturated fractures by water. These simulated experiments focus on cases where covariance (COV), the ratio of the mean of the aperture field to the standard deviation of the aperture field) as well as when the fracture is inclined or declined from horizontal. Results show that when COV is greater than 0.1, then DNAPL will always remain in the fracture after waterflooding. Furthermore, fracture angles below -15 degrees permit the complete removal of DNAPL, while fractures oriented at higher angles do not.

In order to study the transport of particles in water saturated fractures, physical experiments measuring the transport of 0.046 um and 0.55 um microspheres were undertaken on fractures where the geometry could be imported into a computer for comparative simulation analysis. Results demonstrated that during advection, particles generally travel at less than the velocity of the surrounding fluid. As well, hydrodynamic effects such as shear were shown to influence the effluent concentrations by increasing dispersion. Finally, the physical geometry of the fracture was shown to influence the particle pathway during transport and can limit the chances of particles adhering to a fracture wall, thus reducing dispersion and increasing peak concentration. The combined results of these studies show that fracture geometry has a significant effect on the mechanisms of transport in saturated fractures.