The role of bridge cracks in hydraulic fracturing

Hydraulic fractures often contain bridges – parts of unbroken rock connecting the opposite faces of the fracture. Traditionally, they are considered as obstacles for fluid flow. What was overlooked is that they could restrict opening of fractures. We show by laboratory experiment that the bridges are capable of holding the final fracture that apparently cuts the whole surface of the sample. We propose to model a fracture with bridges as a crack with Winkler layer whose stiffness is controlled by the bridge geometry and density. The bridges only affect large fractures, i.e. fractures whose size is of the order of the characteristic scale of the bridge constriction; the latter is inversely proportional to the equivalent bridge stiffness. For such fractures the presence of bridges makes the fracture opening and the stress intensity factors bound as the fracture dimensions proportionally increase. This is in a marked difference from the conventional models according to which both opening and the stress intensity factors tend to infinity as the fracture dimensions increase. The finding can affect the method of design and monitoring of extensive hydraulic fractures and thus reduce the hazards associated with fracking.