Understanding Particle-Particle Interaction during Settling under Gravity

Hydraulic fracturing is a leading technology in natural gas extraction from low permeability reservoirs. Proppants entrained in fracturing fluid stabilize the fracture and prevent premature closures. Therefore, proppant placement and settling characteristics greatly impact fracture conductivity. One key measure of reservoir permeability lies in proppant concentration at fractures. Despite its importance, minimal attention had been given to understanding the complex interactions of two proppant particles settling in static conditions. Generally speaking, an inverse correlation is known between fluid viscosity and a single particle settling velocity, but the correlation between horizontal velocity developed by particle-particle proximity during settling and settling velocity is yet of ambiguity. The present study utilizes spherical glass silica beads to investigate particle-particle settling behavior in a Hele-Shaw cell setup and questions if the quantified horizontal velocity component influences the vertical velocity component of interest. Moreover, the investigated attraction/repulsion phenomenon can unfold new characteristics impacting proppant volumetric concentration during transport. The hydrodynamic interaction of two settling particles was modeled by releasing at varying initial proximities in water. A mechanical release system was utilized to achieve high precision below surface particle release. High-speed particle image velocimetry was conducted, allowing for sophisticated particle tracking in space and time. Preliminary results of particles settling in water indicate a strong dependency of repulsion on initial proximity. Future work includes investigating repulsion characteristics as a function of rheological properties, surface properties and wall effects.