Emergence of Anomalous Transport in Stressed Rough Fractures

Abstract:

Fluid flow and tracer transport in fractured rock controls many natural and engineered processes in the geosciences, and therefore has been extensively studied. Geologic fractures, however, are always under significant overburden stress. While confining stress has been shown to impact fluid flow through rough-walled fractures in a fundamental way, studies of anomalous tracer transport at the scale of individual fractures have so far ignored the potential role of confining stress.

Here, we report the emergence of anomalous (non-Fickian) transport through a rough-walled fracture as a result of increasing the normal stress on the fracture. We generate fracture surfaces with fractal roughness, and solve the elastic contact problem between the two surfaces to obtain the 3D fracture geometry for increasing levels of normal stress. We then simulate fluid flow and particle transport through the stressed rough fracture. We observe a transition from Fickian to anomalous transport as the normal stress on the fracture increases.

We show that the origin of this anomalous transport behavior can be traced to the self-organization of the flow field into a heterogeneous structure dominated by preferential channels and stagnation zones, as a result of the larger number of contacts in a highly stressed fracture. We also propose a spatial Markov model that reproduces the transport behavior at the scale of the entire fracture with only three physical parameters. Our results point to a heretofore unrecognized link between geomechanics and anomalous particle transport in fractured media. Finally, we show preliminary laboratory experiment results that confirm our findings.

(a) Magnitude of the volumetric flux at each discretization grid block at low stress. (b) Magnitude of the volumetric flux for a highly stressed fracture. Values are normalized with the mean volumetric flux.