This paper presents results of an experimental study of the micromechanical proppant behavior during settling in a narrow smooth fracture. This fundamental analysis seeks to better understand dynamics of particle interactions in a dense phase slurry on a small particle size scale, representing proppant settling in narrow hydraulic fractures. Particle Image Velocimetry (PIV) is used for analysis of velocities of individual particle and group particles and their relative paths, collisions and agglomerating in viscous Newtonian fluid. The displacement vectors show the movements of group of particles and global velocity trends of the observed area. The results from this experimental study indicate dependency of settling velocity on particle size and shape, as well as the dependency of different size of particle or agglomerate particles. The slurry settling velocity depends on the relationship between settlement, particle concentration and occurrence of particle agglomeration. The measured results, including vertical velocities and displacement vectors of singular particle and agglomerated particles, were compared with previously published theoretical and empirical relationships. It can be seen that forming of particle agglomerates during settling, caused by frequent particle-particle and particle-wall collisions and interactions, changes the overall settling velocities predicted by the previous experiments in larger slots.
Proppant flow and transport plays a significant role in hydraulic fracturing of georeservoirs. Proppant is small granular material which is placed into hydraulic fracture. Proppant keeps the fracture under high in-situ reservoir stress open for fluid circulation. In geothermal reservoirs, water is continuously cycled through propped fractures, while in oil and gas reservoirs permeable fractures permit production. Experimental, numerical and theoretical research has been conducted for better understanding proppant placement in hydraulic fractures. Trykozko (2016) applied a computational approach to model proppant packs in fractures and confirmed that it causes reduction in the productivity of a hydraulically fractured reservoir. Hammond (1995) did numerical analysis on the gravity-driven vertical motion of proppant in a hydraulic fracture. As a conclusion, while the ratio of facture width to diameter of sand particle is small, the effect of the wall cannot be neglected. Joseph (1994) focused on the different effects of particle-wall interaction and particle-particle interaction both in Newtonian and viscoelastic fluids. He concluded that while particles are close to a vertical wall, the wall will attract particles in a viscoelastic fluid and will dissociate particles in Newtonian fluid. Feng et al. (1994) also investigated the effects of channel wall. The author focused on sedimentation of a single particle with wall effects and concluded that the particles were separated from the wall in a constant distance in Newtonian fluid and the drag reduction can be found on the particles. Gadde et al. (2004) established models for proppant settling in hydraulic fractures and found an empirical correlation which takes particle concentration into account based on Stokes’ law. Liu (2005) developed an experimental correlation between proppant settling velocity and dimensionless fracture width. Malhotra and Sharma (2012) performed an experimental study on the settling velocity of spherical particles in both unbounded and confined walls in shear thinning viscoelastic fluids. The authors found that the walls affected particle settling in viscoelastic fluid, with increasing significance as the ratio of particle diameter to spacing between the walls increases. Additionally, some particle agglomerations were observed during proppant settling between parallel walls. Joseph (1994) studied particle-particle interactions and observed correlations between viscoelastic fluid normal stress and particle agglomeration. Tomac and Gutierrez (2014) studied numerically proppant agglomeration in viscous fluid and the aggregation wasrelated to fracture width and fluid viscosity. It has also been found that particle concentration affects the particle settling velocity. Eskin and Miller (2008) observed that the slurry flow in a fracture is characterized by nonuniform solids concentration across the fracture width. Roy et al. (2015) conducted both experimental and numerical analysis on proppant transport and concluded that the lowest concentration experiment ultimately had the lowest settling velocity.