Hydrodynamic Erosion Phase Diagram and Bulk Permeability Evolution in Filled Rough-Walled Fractures

Natural fractures are commonly filled with materials such as sediments and mineral cements. Under hydrodynamic conditions, these infillings may be scoured, eroded, or removed, leading to alterations in pore structure and bulk permeability. However, the mechanisms driving these changes, particularly the interactions between hydrodynamic conditions, particle migration, and permeability evolution, remain insufficiently understood. In this study, we conducted a series of visual hydrodynamic erosion experiments on fully-filled, rough-walled fractures with varying apertures and roughness characteristics. Using well-calibrated image monitoring and processing techniques, we tracked the erosion process in real time and quantified the resulting eroded flow channels. The results identify five distinct stages across the entire erosion process: particle incipient motion, erosion initiation along with channel penetration, erosion acceleration, deceleration, and depletion. The compiled phase diagrams indicate that the Reynolds number plays a decisive role in erosion dynamics, with fracture aperture serving as the primary geometric control while roughness having a comparatively weaker impact under the identical hydrodynamic condition. We further developed two phenomenological models to predict the variations of erosion ratio and bulk permeability throughout the erosion process. These models capture the effects of Reynolds number, aperture, and roughness on the initiation, growth, and stabilization of erosion and permeability changes. These findings offer a deeper understanding of how hydrodynamic forces drive erosion in complex fracture systems and provide valuable insights into various fields concerned with the coupled issues of seepage and erosion.