We performed laboratory scale experiments for the study of horizontal fluid driven fracture initiation from an open-hole section. In order to clarify the role of material microstructure and fluid infiltration, we used viscous Newtonian fluids to initiate penny shaped fractures from radial notches in blocks of natural sandstone. We varied the fluid viscosity and injection rate over ranges allowing for significant fluid losses toward the dry porous rock. For low efficiency fluids, the fracture propagation was characterized by non-monotonic pressure variations. We propose a formulation of the problem of hydraulic fracture initiation in an elastic homogeneous permeable material. Our model accounts for fluid losses toward the rock and a cohesive zone at the fracture tip. We performed dimensional analysis in order to lump the physical parameters into meaningful dimensionless groups. Scale effects concerning the fluid efficiency, decompression effects and material failure could be predicted from dimensional analysis and corroborated with experimental observations. We also propose a numerical solution for this model. The numerical results compare well with experimental results. This work provides a proper scaling for hydraulic fracture initiation. This scaling is necessary for comparing laboratory scale results to field observations.

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