Fracture criteria at the tip of fluid‐driven cracks in the Earth

The effect of high confining pressure on fluid‐filled crack growth is considered. Exact solutions are given for a two‐dimensional horizontal crack in an infinite elastic body using the approximation of Dugdale‐Barenblatt (DB) model. It is shown that for equilibrium cracks (i.e. for cracks on the verge of propagation) the large‐scale crack characteristics, such as fluid overpressure, apparent fracture toughness, maximum opening of the crack and crack volume, grow with increase of confining pressure. These effects result from a pressure induced fracture resistance (PIFR). If basic parameters of the DB model (tensile strength and critical crack opening displacement) are independent of confining pressure then PIFR dominates over intrinsic rock strength starting from quite shallow depth (tens to hundreds of meters).