In fluid driven fractures, energy is dissipated by creating new fracture surfaces and overcoming viscous resistance to fluid flow. It is known that when fluid driven fractures propagate in a rock with a finite amount of resistance to fracture, the toughness (?1/2) and viscosity (?2/3) solutions for the fracture aperture co-exist near the fracture tip. Depending on the ratio of viscous dissipation and fracture resistance, the region for the validity of each tip asymptote varies. In this paper, we developed a 2D displacement discontinuity based hydraulic fracture propagation model for multiple large-scale hydraulic fracture propagation while taking into account the multi-scale nature of the fracture tip. The numerical model is capable of distinguishing between viscosity and toughness dominated propagation regimes, and takes into account their corresponding solutions as boundary conditions near the fracture tips. We provide numerical examples of large-scale multiple fracture propagation in a rock with finite fracture toughness using injection fluid viscosities range from 1-1000 cP. The results include fracture geometry, aperture distribution and regime of fracture propagation. To study the effect of fracture tip asymptote on the resulting fracture cluster, we approximated multi-scale fracture tip asymptotes with toughness asymptote showing that for a rock with finite toughness and for the range of fluid viscosities considered, the toughness solution provides sufficiently accurate results irrespective of the regime of fracture propagation.

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