Escape of fluid-driven fractures from frictional bedding interfaces: A numerical study

Fluid-driven or hydraulic fractures, either natural or man-made, which are propagating vertically in horizontally layered rocks, may interact with and deflect into bedding interfaces that they intersect. Pre-existing secondary flaws located along the frictional bedding contact defined as the bedding plane, can serve as nucleation sites for new fractures that are extended first by interface slip and then by fluid. If the flaw propagates, a stepped fracture forms. In this paper, a two-dimensional numerical fracture model is used to analyse the coupled rock deformation and fluid flow in such fractures. Numerical results show that fracture escape from the interface is likely under the conditions of fracture growth from stiff to soft rocks, small layer-to-layer far-field stress contrasts, and moderately low fluid viscosity, and small parent fracture lengths and offset distances. The change of the fracture propagation direction at the bedding plane gives rise to a variety of fracture and fluid flow patterns. Fracture permeability and internal pressure vary with time and location. Vertical fracture growth can impede fluid flow along the offset by inducing additional compressive stress on it. If the offset channel is of relatively large permeability, most fluid will be directed into the new fracture to facilitate fracture escape. Alternatively, the interaction and geometry of the fracture branches can lead to low opening compliance on the offset and localised pinching of the fracture surfaces, which restrict fluid flow.