**4. Conclusions**

Important applications of injection of a low viscosity fluid into a pre-existing fracture, such as waterflooding and supercritical CO2 injection in geological sequestration, necessitate comprehensive studies of mechanical and hydraulically properties of fractures from the beginning of injection until the onset of fracture propagation. In this study, we considered a low viscosity fluid injection into a pre-existing, un-propped crack of a PKN geometry. We focus on the case of a critically-overpressured reservoir and initially closed (un-propped) crack. The extension of this work to propped cracks and more general reservoir conditions are reported elsewhere.

The analysis assumes negligible viscous dissipation during injection of a low viscosity fluid at a sufficiently slow injection rate [22], and, as a result, approximately uniform pressure distribution in the crack. Furthermore, the poroelastic effects are also neglected in this study. To outline the validity of the latter assumption, we can show that the later stages of transient pressurization (the so-called leak-off dominated regime when the injection time ≫ diffusion timescale ℓ2/4*α*) with and without poroelasticity effects are identical. However, the generated poroelastic backstress which tends to close the fracture may cause a delay in the initiation of crack propagation when compared to the case where poroelastic effects are neglected. In addition, for certain ranges of fracture and fluid properties and field operating condition the backstress may become large enough to prevent the fracture from propagation indefinitely.

We evaluated the evolution of the fluid pressure inside the fracture during the transient pressurization by considering 2-D fluid diffusion from the fracture into the surrounding porous rock. As the fracture is pressurized, the condition for the onset of its propagation (breakdown condition) is eventually reached. We quantified how the fracture breakdown condition depends upon the rock and fluid properties, the in-situ stress and the fluid injection rate. The history of the transient pressurization prior to breakdown can be used to provide the initial conditions for the fracture propagation problem.
