Response of Natural Fractures to Hydraulic Fracturing

Abstract:

We extend a dynamic earthquake rupture simulation code to model propagation of both shear and tensile fractures and seismic waves radiated from these fractures. We particularly aim to explore how pre-existing natural fractures respond to hydraulic fracturing treatment. We use split nodes to define fractures in the finite element mesh. By appropriately implementing jump conditions on the two different types of fractures, we can simulate dynamics propagation of shear and tensile fractures using the finite element code. Shear failure is governed by the Mohr-Coulomb criterion and a slip-weakening friction law dictates friction evolution on shear fractures during shear sliding. Tensile failure occurs when the effective normal stress acting on a fracture reaches tensile strength. In our models, pressurization and propagation of a hydraulic fracture due to fluid injection drive deformation.

Our preliminary results show that natural fractures that are parallel or perpendicular to the hydraulic fracture, which are commonly assumed in many hydraulic fracturing models, are unlikely to be activated by hydraulic fracturing, if no natural fractures in other orientations exist. However, activation of natural fractures that are optimally oriented in the initial stress field may trigger sliding or opening of natural fractures in other orientations. These results suggest important roles both static stress and dynamic stress play in response of natural fractures to hydraulic fracturing.