Hydraulic Fracture Propagation in Pre-Fractured Natural Rocks

Abstract We conducted a series of hydraulic fracture initiation tests with fractured natural rock samples. The objective is to characterize the interaction between hydraulic fracture initiation and natural fracture infiltration and opening. The natural fracture was simulated by cleaving a rock cube into two or three layers and putting the parts back together. The hydraulic fracture is initiated by pressurizing the borehole drilled perpendicular to the layers. The test parameters include rock type (sandstones and a tight limestone), confining stress and fluid type (silicon oil and cross-linked gel). A detailed model was built to simulate the hydraulic fracture interaction with a natural fracture. Qualitative agreement was found with the test results. Introduction Natural fractures come in many different forms. As one extreme they may be just a weakness governed by the rock fabric, another extreme is a very conductive and wide open shear zone. Since shear dilatancy is not only occurring on a microscopic scale but also on macro-scale by fault block rotation, shear can generate very high conductivity along fault zones. If a hydraulic fracture grows into such a fault with a large aperture, it is likely that the fluid will flow into the fault. This is often the objective in geothermal reservoirs where hydraulic fractures are needed to connect the well to shear zones that produce large amounts of water