Cross-scale proppants transport in the shale hydraulic fracture network: A hybrid CFD-DEM investigation

In hydraulic fracturing operations within layered formations such as shale, branching fractures are easily formed. The transport and placement of multi-sized proppants within these fractures play a vital role in maintaining post-frac permeability and ensuring long-term reservoir productivity. However, characterizing hydraulic fractures in layered formations and modelling the complex transport behaviour of cross-scale proppants, ranging from large to micro-sized particles, remains a significant challenge in geological and reservoir engineering. To address this, We developed a novel hybrid CFD-DEM model to overcome limitations of conventional CFD-DEM schemes, especially at extreme mesh-to-particle size ratios. Our model employs an unresolved CFD-DEM approach for mesh-to-particle ratios of 2 or higher and an improved semi-resolved CFD-DEM technique for ratios ranging from 0.1 to 2. An enhanced smoothing strategy was integrated into the semi-resolved approach to address insufficient smoothing space near fracture walls. A representative layered shale fracture network, featuring multiple horizontal bedding-plane fractures, was constructed to study the influence of bedding plane quantity and roughness on proppant transport. Our results demonstrate that upward flow conditions favour the entry of larger proppants into bedding-plane fractures, whereas downward flow conditions promote the diversion of micro proppants. Furthermore, we identified that the vertical velocity of regular proppants significantly governs their diversion into bedding-plane fractures, while fluid forces at fracture intersections are the primary control for micro proppant diversion. This study provides groundbreaking insights into the transport dynamics of multi-sized proppants in layered shale fracture networks, offering practical implications for optimizing hydraulic fracturing designs in stratified geologic formations. • A novel hybrid CFD-DEM approach was developed and validated. • An unprecedented study on cross-scale proppants transport was conducted. • A realistic hydraulic fracture network in layered formations was constructed. • Two distinct proppant transport behaviours and their mechanisms were identified.