Experimental investigation of particle settling characteristics in narrow fractures with rough walls

Particle settling in narrow rough fractures is a common but poorly understood phenomenon during hydraulic fracturing. This study first constructs a large slot with two rough surfaces to simulate rock fractures and employs the particle image velocimetry to measure particle settling. Results show that particle settling in the rough slot is more complex than in the smooth slot. Rough pathways significantly change particle settling characteristics. The rough-walled slot alters the classic settling process by creating preferential pathways, localized trapping, and vortex-driven redistribution. Particles settle along preferential pathways, increasing settling velocity. The wall retardation effect becomes more prominent for large particles, reducing the settling velocity. Particle settling induces vortices throughout the rough surface, affecting particle behavior. Higher particle volume fractions increase settling non-uniformity, leading to unstable fluid flow within fractures, characterized by high vorticity and upward flow. The frequent interplay between particles and particle-walls, and fluid resistance complicates particle trajectories and settling behavior. Fluid viscosity significantly changes settling patterns and promotes particle clusters, forming chain-like and curtain-like clusters in rough fractures. An innovative model is proposed to predict settling velocity in rough fractures.