Radial flow in an anisotropic aperture field in a rough-walled fracture

Preferential flow channels with low hydraulic resistance in a single fracture can transmit large fluid volumes in fractured rock. Characterizing these channels and the associated flow channeling at the field scale, however, remains challenging. In this study, a hybrid iterative computational workflow was used to generate a field-scale (30 m diameter) synthetic fracture with self-affine surfaces and a high-resolution (2 mm spatial resolution) aperture field. We investigate the effects of anisotropic surface roughness, shearing displacement, and closure ratio on flow channeling in a radial flow system using advective Lagrangian particle tracking and advection–dispersion numerical simulations. Results show a transition from local microscale channeling effects to a homogenous flow pattern at the field-scale within rough-walled fractures. The transition is enhanced by the increasing microscale channels which occur via surface roughness anisotropy. Surface shearing displacement and anisotropic surface roughness control the fracture closure area and principal orientations, aligning with the tortuosity and direction of preferential flow pathways. The solute peak arrival times in synthetic fractures are similar to those in smooth fractures with an equivalent Cubic Law aperture. This demonstrates the applicability of the Cubic Law aperture in predicting bulk arrival times, even in the presence of flow channeling within sheared synthetic fractures. The strong flow channeling behavior observed in some field studies is therefore likely related to local geological conditions (stress included) and post-fracturing processes, underscoring the need to integrate initial fracturing and subsequent weathering history obtained using rock sample properties and outcrop analysis when investigating channeling effects.