Anisotropy of tracer dispersion in rough model fractures with sheared walls

TL;DR

This study investigates how the orientation of shear displacement in rough model fractures affects tracer dispersion, revealing anisotropic mixing behaviors and the influence of fracture structure on flow and dispersion patterns.

Contribution

It provides new insights into the anisotropic dispersion of tracers in sheared fractures, highlighting the impact of shear orientation on flow structures and mixing front morphology.

Findings

Displacement fronts show finger-like patterns when shear is perpendicular to flow.

Front flatness and dispersivity distribution depend on shear orientation.

Flow and dispersion are well explained by channel models based on aperture fields.

Abstract

Dispersion experiments are compared for two transparent model fractures with identical complementary rough walls but with a relative shear displacement $\vec{\delta}$ parallel ($\vec{\delta}\parallel \vec{U}$) or perpendicular ($\vec{\delta} \perp \vec{U}$) to the flow velocity $\vec{U}$. The structure of the mixing front is characterized by mapping the local normalized local transit time $\bar t(x,y)$ and dispersivity $\alpha(x,y)$. For $\vec{\delta} \perp \vec{U}$, displacement fronts display large fingers: their geometry and the distribution of $\bar t(x,y)U/x$ are well reproduced by assuming parallel channels of hydraulic conductance deduced from the aperture field. For $\vec{\delta} \parallel \vec{U}$, the front is flatter and $\alpha(x,y)$ displays a narrow distribution and a Taylor-like variation with $Pe$.