A review of experimental studies on the proppant settling in hydraulic fractures

The hike of hydraulic fracturing in North America and beyond leads to significantly enhanced hydrocarbon production especially in low-permeability geological formations. Proppant settling is one of the most important particle motions during a hydraulic fracturing process, which to a large extent determines the created fracture's conductivity. This paper provides a critical review of the proppant settling in hydraulic fractures, with an emphasis on the experimental studies published in the past several decades. Six factors are identified out of a proppant/fracturing-fluid/fracture system: wall retardation, fluid rheology, proppant non-uniformity, proppant surface wettability, proppant concentration, and fracture complexity. Influences of these factors on the proppant settling are summarized based on the analysis of published experimental data, images, and correlations. Notwithstanding the extensive experimental studies, more experiments are in demand to systematically investigate proppant settling behavior in various kinds of fracturing fluids confined by hydraulic fractures. In addition, a gap is found to exist between the experimental studies of proppant transport and the application of the experimental findings to numerical simulations of hydraulic fracturing.