INCORPORATING IRREGULAR FRACTURE GEOMETRY IN THE STUDY OF VARIABLE-DENSITY GROUNDWATER FLOW AND SOLUTE TRANSPORT IN DISCRETELY-FRACTURED MEDIA

Variations in fluid density can greatly affect fluid flow and solute transport and must be accounted for, among other hydrogeological situations, seawater intrusion, radioactive waste disposal, or inland salinity. Heterogeneities in the properties of geologic materials play a major role for the migration of variable-density fluids. In a modeling study, Shikaze, Sudicky and Schwartz (1998, JCH) have shown that, for regular orthogonal fracture networks embedded in a porous matrix, dense solute plumes may develop in a highly irregular fashion and the uncertainty associated with prediction can be very high. However, other recent studies have shown that the style of heterogeneity in a porous medium will greatly influence the propagation of dense plumes, with disorganized heterogeneity tending to dissipate convection through mixing and thus reducing plume instabilities. Therefore, it still remains unclear if dense plume instabilities will be as pronounced for fractured networks lacking a regular pattern than for regular fracture networks.

To address this question, the FRAC3DVS model, which solves 3D variably-saturated flow and solute transport in discretely-fractured porous media, has been modified to account for density variations. A general formulation of the body force vector is derived such that variable-density flow and transport is accounted for fractures of any arbitrary inclination, relaxing the assumption of either vertical or horizontal fractures. Assuming that arbitrary fracture inclination enhances the level of heterogeneity disorganization, the model allows investigating scenarios of plume migration in more disorganized media than done previously. Simulations are presented that show the verification of the new model formulation, for inclined fractures and for the porous matrix. Simulations of variable-density flow and solute transport are then conducted for irregular fracture networks, to investigate if they indeed tend to smooth out plume migration. Variations in the hydraulic properties of the fractures and the porous matrix are also considered in the study. Finally, some of the numerical difficulties associated with such simulations are discussed.