12 13 For hydraulic fracturing design in unconventional reservoirs, the existing proppant transport 14 models ignore the fluid leak-off effect from the fracture side wall and the effect of fracture 15 roughness. In this paper, a model is proposed using three-dimensional computational fluid 16 dynamics approach with fluid leak-off rate defined along the fracture length and considering 17 the effect of fracture roughness on proppant distribution. Based on the simulation results, it is recommended that neglecting the fracture roughness in the proppant transport model can result in over predicting the proppant bed length and underpredicting the proppant suspension layer by 10-15%. Furthermore, neglecting the fluid leak-off effect can result in under predicting the proppant bed height by 10-50% and over predicting the proppant suspension layer by 10-50%. This study has enhanced the understanding of the proppant-fracturing fluid interaction phenomenon by accounting detailed physics to optimise the hydraulic fracturing design. Keywords Proppant transport; Hydraulic fracturing; Computational Fluid Dynamics; Discrete Element Method; Fluid Leak-off; Fracture Roughness Highlights-• Proppant transport in rough fractures with fluid leak-off from fracture wall • Parametric study of proppant properties, fluid properties, and fracture properties • Effect of using foam (Non-Newtonian) fracturing fluid Graphical Abstract-1. The advancements in the multistage hydraulic fracturing technology have resulted in the considerable progress in the hydrocarbon production in the last decade (Lange et al., 2013; Li et al., 2015; Yuan et al., 2018). Hydraulic fracturing is a technique in which fractures are initiated and propagated due to the injection of highly pressurised fluid at sufficiently high rates in the subsurface reservoir (Donaldson et al., 2014). When the fracture is estimated to be sufficiently long and wide, sand or other suitable material called proppants are injected with the additional fluid, to keep the fractures open against the rock pressure (Yew and Weng, 2014). The hydraulic fracturing in unconventional reservoirs is significantly different from the conventional reservoirs mainly because of the two reasons. Firstly, in conventional reservoirs, the focus of the hydraulic fracture design is to have a large fracture width, whereas, in the low permeability unconventional reservoir, greater fracture length is the prime factor to optimise (Belyadi et al., 2016). Secondly, slick water is commonly used as a fracturing fluid in the unconventional reservoir and due to the low viscosity of slick water and negligible chemical additive, tendency to suspend the proppant significantly decreases (Sahai et al., 2014). This results in early proppant deposition compared with conventional fracturing fluids (Alotaibi and Miskimins, 2015). Therefore, both of these attributes for the unconventional reservoirs, i.e. focus is on creating a longer fracture and early deposition of the proppants, result in closing of the unpropped section of the fr...

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Numerically modelling the fluid flow with proppant transport and fracture propagation together 11 are one of the significant technical challenges in hydraulic fracturing of unconventional 12 hydrocarbon reservoirs. The existing models either model the proppant transport physics in 13 static predefined fracture geometry or account for the analytical models for defining the fracture 14 propagation. Furthermore, the fluid leak-off effects are usually neglected in the hydrodynamics 15 of proppant transport in the existing models. In the present paper, a dynamic and integrated 16 numerical model is determined that uses computational fluid dynamics (CFD) technique to 17 model the fluid flow with proppant transport and Extended finite element method (XFEM) to 18 model the fracture propagation. The results of fracture propagation were validated with the real 19 field results and analytical models, and the results of proppant transport are validated with the 20 experimental results. The integrated model is then used to comprehensively investigate the hydrodynamical properties that directly affect the near-wellbore stress and proppant distribution inside the fracture. The model can accurately model the proppant physics and also propose a solution to a frequent challenge faced in the petroleum industry of fracture tip screen out. Thus, using the current model allows the petroleum engineers to design the hydraulic fracturing operation successfully, model simultaneously fracture propagation and fluid flow with proppant transport and gain confidence by tracking the distribution of proppants inside the fracture accurately. Keywords Hydraulic fracturing, XFEM-based cohesive law, Computational Fluid Dynamics, Proppant transport; Fluid leak-off; Fracture propagation; Fracture tip screen-out Highlights • Proppant transport model with fluid leak-off and dynamic fracture propagation • Fluid flow modelled using CFD-DEM hybrid model and propagation using XFEM model • Results validated with real field data, analytical model and experimental study • Effect of injection rate, fluid viscosity and leak-off rate investigated • Investigated the parameters to mitigate fracture tip screen-out Graphical abstract