Grid re®nement for modeling multiphase ̄ow in discretely fractured porous media

A study of the e€ects of grid discretization on the migration of DNAPL within a discrete-fracture network embedded in a porous rock matrix is presented. It is shown that an insuciently ®ne discretization of the fracture elements can lead to an overprediction of the volume of DNAPL that continues to migrate vertically at the intersection of a vertical and horizontal fracture. Uniform discretization of elements at the scale of one centimetre (or less) accurately resolved the density and capillary pressure components of the head gradient in the DNAPL. An alternative, non-uniform method of discretization of elements within the discrete-fracture network is presented whereby only fracture elements immediately adjacent to fracture intersections are re®ned. To further limit the number of elements employed, the porous matrix elements adjacent to the fracture elements are not similarly re®ned. Results show this alternative method of discretization reduces the numerical error to an acceptable level, while allowing the simulation of ®eldscale DNAPL contamination problems. The results from two ®eld-scale simulations of a DNAPL-contaminated carbonate bedrock site in Ontario, Canada are presented. These simulations compare di€erent methods of grid discretization, and highlight the importance of grid re®nement when simulating DNAPL migration problems in fractured porous media. Ó 1999 Published by Elsevier Science Ltd. All rights reserved.