Micro-Proppant Transport in 3D-Printed Rough Natural Fractures Using Time-Resolved Particle Image Velocimetry

Abstract Unconventional oil and gas reservoirs contain numerous secondary natural fractures (<1 mm), making it difficult for conventional- sized proppants to enter these fractures, resulting in a low effective stimulated reservoir volume. Micro-proppants (particle size<200 mesh) have the potential to enter micron-scale fractures, activate secondary fracture networks, and enhance hydro- carbon recovery. However, limited experimental studies have visualized the proppant transport behavior in fractures with dual roughness on both surfaces. In this study, a Time-Resolved Particle Image Velocimetry (PIV) based experimental setup was employed to investigate the transport and placement of fluorescent-coated micro-proppants in rough fractures. Synthetic fracture surfaces with controlled fractal dimensions and RMS roughness values were generated using SynFrac software and fabricated via 3D printing. A systematic investigation was conducted to assess the effects of proppant concentration, injection rate, and fracture dip angle on micro-proppant placement. Pressure sensors were used to quantify micro-proppant transport dynamics in rough secondary fractures and to identify key parameters affecting placement efficiency. Experimental results show that increasing fracture inclination significantly influences micro-proppant transport and de- position. As the fracture angle increases, the deposition pattern shifts from cluster-based blockage to dune-like accumulation, reducing placement efficiency in the distal region. Higher proppant concentrations enhance interparticle interactions, leading to aggregation and bridging, which limit transport distance and hinder effective placement in narrow fracture sections. In- creasing injection rate promotes deeper transport of micro-proppant. However, when the rate exceeds a critical threshold (Re = 16.42), excessive proppant migration toward the fracture tip reduces deposition near the inlet, resulting in uneven distri- bution. Therefore, the interaction between proppant concentration and injection rate jointly controls placement patterns and efficiency. Optimizing these parameters can significantly improve micro-proppant distribution and placement performance within the fracture. These findings provide new insights into micro-proppant transport and placement in rough-walled fractures, contributing to the optimization of stimulation strategies for unconventional reservoirs.