A new CFD approach for proppant transport in unconventional hydraulic fractures

Abstract

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...

Proppant transport in dynamically propagating hydraulic fractures using CFD-XFEM approach

“…The equations describing the hybrid model for proppant transport used in the current study is explained in detail in our previous work. 38 However, the key governing equations are briefly described as follows. For an isothermal condition and incompressible fracturing fluid, the mass conservation equation is given by:…”

“…The drag force is described by the Eq. (38). Numerous drag force models are available for multiphase flow modelling that differs in the definition of inter-phase momentum exchange coefficient, K ls or K sl .…”

“…ℯss = 0.9 representing inelastic collision is used in the present study. 38 αs,max is the maximum packing limit for the particles. In the present study, a maximum packing limit of 0.63 is used.…”

“…θ = 30 0 is used in the present study. 38 Johnson and Jackson 66 proposed a model to calculate friction pressure given in Eq. (52).…”

“…However, in the present paper, the proppant transport and fluid flow are modelled 150 solving the flow governing equation for both the phases individually and the proppant-fluid 151 interaction is explicitly modelled using Hybrid Model (CFD-DEM). 38 The model was then integrated to couple the effect of dynamic fracture propagation with the fluid leak-off effects.…”