DISCRETIZING NON-PLANAR DISCRETE FRACTURES FOR 3D NUMERICAL FLOW AND TRANSPORT SIMULATIONS

A common problem for 3D numerical simulations of fluid flow and mass transport in fractured porous media is the realistic representation of 2D discrete fractures in 3D finite element grids. If orthogonal grids are used, discrete fractures are usually aligned on grid lines, allowing only for representation of orthogonal fractures. Non-orthogonal inclined fractures have previously been discretized on orthogonal grids in a staircase fashion, by combining horizontal and vertical fracture elements. This method however increases the actual fracture lengths and can influence simulation results such as underestimating solute concentrations because flow paths are lengthened. Non-orthogonal irregular grids and distorted elements can also be used to discretize inclined fractures in 3D. In this case, the grid geometry must be chosen such that node layers coincide with fracture locations. This method has the disadvantage that discretizing fracture networks of high complexity is very challenging, time-consuming and sometimes impossible. Describing discrete fractures within a 3D grid is further complicated when fractures are non-planar.

To simplify discretizing non-planar inclined discrete fractures, the FRAC3DVS model (Therrien and Sudicky, 1996, J Contaminant Hydrol), which solves 3D variably-saturated flow and solute transport in discretely-fractured porous media, has been modified. The new model triangulates non-planar inclined fractures and represents each triangle by a series of rectangular and triangular fracture elements, which can be horizontal, orthogonal or inclined. With the new model, a single inclined fracture has been discretized and flow/transport simulations were conducted in 2D and 3D. The results were compared with simulations where the inclined fracture is represented using orthogonal elements. Simulations indicate that (i) steady-state flow results of both methods are identical, (ii) hydraulic heads of transient flow results are underestimated when orthogonal elements are used, (iii) concentrations are underestimated when orthogonal elements are used and, (iv) concentrations are identical when flow velocities in orthogonal fracture elements are corrected to account for longer flow paths. The proposed discretization procedure offers new possibilities to simulate flow and transport in complex 3D fracture networks.