Optimizing proppant placement in unconventional reservoirs: The critical role of fracture roughness anisotropy.

Hydraulic fracture roughness often exhibits distinct scales across different spatial directions, yet the impact of this anisotropy on proppant transport remains poorly understood. To address this gap, we developed an experimental platform that integrates synthetic fracture morphology with high-resolution 3D printing to fabricate surfaces with precisely controlled anisotropy factors. By systematically varying roughness anisotropy, fracture aperture, injection pressure, proppant size, and flow orientation, we evaluated their combined effects on transport behaviour and dynamic permeability. Unlike isotropic fractures that suffer from severe inlet clogging, anisotropic fractures significantly enhance proppant intake and coverage. These benefits scale with moderate anisotropy (anisotropy factor ≤ 4) and plateau beyond a critical threshold (anisotropy factor > 8). Mixed-size injections exhibit a “roughness sieving” mechanism where transverse ridges arrest large particles into a permeable skeletal framework, allowing fines to navigate interstices and fill distal voids. Furthermore, we identified an optimal aperture “sweet spot” at 2.15 mm, representing an aperture-to-particle size ratio of approximately 3.4 for 20/40 mesh proppants. Wider apertures lower transport efficiency due to insufficient fluid drag required to scour proppants over roughness ridges. Higher injection pressures enhance distribution uniformity but decrease permeability through the hydrodynamically induced compaction of proppants. Finally, horizontal fractures show “roughness skipping”, where the lack of gravitational locking allows particles to flow unimpeded through transverse valleys and glide over ridges, leading to high washout rates relative to vertical orientations. These findings demonstrate that tailoring pumping strategies are required to exploit these special transport behaviours in anisotropic rough fractures for optimized fracture conductivity.