On the Stability of Particle-Particle Interaction during Gravitational Settling

The elevated energy demand and high dependency on fossil fuels have directed researchers' attention to promoting and advancing hydraulic fracturing (HF) operations for a sustainable energy future. Even though previous studies have demonstrated that the proppant suspension and positioning in slickwater play a vital role during the shut-in stage of the HF operations, minimal experimental work has been conducted on the fundamental proppant-proppant interaction mechanisms, especially a complete mapping of the interactions. This study utilizes high-speed imaging to provide a 2D space- and time-resolved investigation of two-particle (proppant models: 2 mm empty set, 2.6 g.cm(-3)) interactions during gravitational settling in different initial spatial configurations and rheological properties. The mapping facilitates the identification of various interaction regimes and newly observed particle trajectories. Pure water results at a settling particle Reynolds number (Re-p) similar to 470 show an unstable particle-particle interaction regime characterized by randomness while altering pure water to a 25% (v/v) water-glycerin mixture (Re-p similar to 200) transitions an unstable interaction to a stable prominent repulsion regime where particles' final separation distance can extend up to four times the initial distance. This indicates the existence of Re p at which the stability of the interactions is achieved. The quantified trajectories indicate that when particles are within minimal proximity, a direct relation between repulsion and Re p exists with varying repulsion characteristics. This was determined by observing unique bottle-shaped trajectories in the prominent repulsion regimes and further highlighted by investigating the rate of lateral separation distance and velocity characteristics. Additionally, a threshold distance in which the particles do not interact (or negligibly interact) and settle independently seems to exist at the normalized 2D lateral separation distance.