Linking of fractures in layered rocks: Implications for permeability

The permeability of many rocks, including many reservoirs, is mostly attributable to fractures that form interconnected clusters. Here we present the results of field studies and numerical models on the linkage of fractures, using primarily fractures in carbonate rocks from the UK. The numerical models focus on two fracture configurations: five offset fractures in a 5-layer model, and a single hydrofracture in a 3-layer model. In some of the 3-layer models, a weak, open contact is added between the two topmost layers. In the 5-layer models loading is by 5 MPa tensile stress, whereas the 3-layer models the only loading is the internal fluid overpressure of 6 MPa in the hydrofracture itself, located in the lowermost of the three layers. For the 5-layer models, tensile stresses occur between the nearby tips of the offset fracture pairs, but these stresses are generally too low in the soft shale layers to initiate fractures. The tensile stresses, however, commonly concentrate at the contacts between the shale and limestone layers and may therefore result in delamination (debonding), that is, opening of the contacts. By contrast, the shear stresses between the nearby tips of the fracture pairs are often high enough to generate shear or mixed-mode fractures that connect the nearby tips of the original offset fractures, resulting in clusters that can conduct fluids. For the 3-layer models, the fracture-induced tensile stresses in the uppermost layer are suppressed by a comparatively thick compliant (shale) layer and/or the weak, open contact. Thus, no fracture formation is possible in that layer but at their contacts with the compliant layer the hydrofracture apertures increase. Thus, a compliant layer may arrest the vertical hydrofracture propagation from its stiff host layer but, at the same time, increase its maximum aperture and encourage lateral fluid transport at the layer contact between the layers.