Simulation of reactive transport in fractured geologic media using random-walk particle tracking method

Abstract

Groundwater flow and contaminant transport through fractured porous media are encountered in many subsurface systems and are thus of great importance in many scientific and engineering fields. The discrete fracture network (DFN) technique is the most representative technique for modelling fractured porous media, but it is computationally very demanding. A random walk particle tracking (RWPT) method is a suitable tool for simulating contaminant transport in discretely modeled fractured systems. In this research work, a particle tracking contaminant transport model, previously developed and verified for simulating advection and dispersion processes in discrete fractures, is adapted to include matrix diffusion and reactive processes. Matrix diffusion is incorporated into the technique using transfer probabilities. Reactive processes occurring in both the fractures and the matrix are incorporated into particle movements and transfer between the different phases. Equations are developed to determine the relation between several controlling parameters to ensure model accuracy. The modified DFN-based model is verified by comparison to available analytical solutions for the case of a single fracture taking into account all processes. The model is also verified with the analytical solution (SOLFRAC) for the case of transport in a DFN considering the sorption process occurring along the fracture walls. Comparisons show good agreement between the developed DFN-based RWPT model and the other solutions. The results indicate that the particle tracking model is performing well and provides an alternative to dual-porosity and dual-permeability models. However, for achieving accuracy and stability, the approach is limited to cases with diffusion coefficient values ranging between 1 × 10–11 and 1 × 10–8 m2/s. Also, the pore velocity values in fractures are limited to a maximum of 10 m/day, and average fracture length should not exceed 100 m.