A poroelastic model for laboratory water injection test in highly permeable rocks

Water injection laboratory experiments in weak, poorly consolidated sandstones show evidence that the peak injection pressure is much larger than the one predicted by the Haimson-Fairhurst criterion. A model based on poroelasticity, fracture mechanics, and lubrication theory is constructed to simulate the laboratory experiments. It aims at computing the propagation of a bi-wing hydraulic fracture from a borehole with increasing injection rate, until the crack reaches the boundary of the sample. The model is applicable to situations for which the pore pressure field reaches steady-state quasi-instantaneously when changing the injection rate, on account of the large permeability of these rocks. Two asymptotic regimes of solution are found: (i) a rock-flow regime where the induced fracture is hydraulically invisible, and (ii) a fracture-flow regime where the fluid penetrates the rock via the crack. In the rock-flow regime, fracture propagation is stable, i.e., the borehole pressure increases with the injection rate, while in the fracture-flow regime, the reverse is true. It is therefore concluded that the peak injection pressure reflects a transition between two flow regimes, rather than breakdown.