Solute Transport in Fractured Geological Formations: Insights from Particle Tracking Simulations

In the context of safety management of subsurface water resources, geometrical properties of fractures, e.g., their aperture and length distributions, play a key role. In subsurface hydrology and petroleum engineering, fractures serve as primary pathways for fluid flow and solute transport, particularly in low porosity and permeability formations. Accordingly, understanding the effect of fracture network heterogeneity on flow and transport is of great importance. In this study, we generated a three-dimensional discrete fracture network whose fracture length followed a truncated power-law (TPL) distribution using the dfnWorks, a computational suite developed by the Los Alamos National Laboratory. We assumed α = 1.5, 2.0, and 2.5, the exponent in the TPL fracture length distribution, and presumed that the fracture aperture and length are correlated. The minimum b0 and maximum b1 fracture apertures were 0.0003m and 0.0025m, respectively, and the minimum l0 and maximum l1 fracture radii were 1.5m and 6.75m. We simulated fluid flow based on the Reynolds equation and solute transport using the particle tracking approach in a fracture network of size 20m at a fracture density of 0.10. Simulations were iterated at least 20 times, and the average arrival time distribution was analyzed. We found that as the exponent α increased from 1.5 to 2.5, the permeability of fracture networks decreased from 2.21 × 10-9 to 1.19 × 10-9 m2. We determined the slope of the averaged arrival time distributions by fitting a power law (i.e., t-1-β) at long time scales. Results showed that as the exponent α increases, the value of β decreases. We found β = 0.88, 0.83, and 0.79 for α = 1.5, 2, and 2.5, respectively. Increasing the exponent α delays the arrival times of the solutes as particles transverse through longer paths due to higher levels of heterogeneities. Further investigations are required to investigate the effect of domain size and fracture density on solute transport in fracture networks.