Fracture roughness effects on slickwater proppant slurry settling attenuation

This study evaluates and quantifies fracture roughness effects on proppant particle settling in fractures utilizing coupled, resolved computational fluid dynamics (i.e., simulation of carrier fluid behavior) and the discrete element method (i.e., simulation of proppant particle behavior). This work focuses specifically on "slickwater" based proppant slurries. Rough fracture geometries are generated using a spectral-based algorithm with surficial roughness varied by adjusting fractal dimension and root-mean-square asperity heights descriptive parameters. This work also considers particle volumetric concentration and influence from flowing conditions on particle settling. Results indicate that the attenuation of quiescent settling rates is more pronounced in rough fractures as the aperture narrows. The degree of quiescent slurry settling attenuation corresponds to fractal dimension and root-mean-square asperity height values. A formulation to describe the average settling rate of quiescently settling slurries in rough fractures is proposed and evaluated. Fracture roughness causes erratic settling velocities of flowing slurries, with some cases having upward proppant velocity, specifically at narrow apertures at or below two times the particle size for the condtions considered in this work. The onset of this behavior is dependent on degree of roughness, with erratic velocities occurring at narrower relative apertures for smoother fractures compared to rougher fractures. Preferential flow pathways formed in rough fractures at narrower apertures, are noted to cause these erratic settling behaviors. The overall findings provide a greater understanding of particle settling behavior in rough rock fractures, aiding in the design process for fracture enhancement projects.