We conducted a series of hydraulic fracture initiation tests with pre-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 made 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 rock types include Colton Sandstone, Felser Sandstone and Pierre Bleue Limestone. Test parameters include confining stress and fluid viscosity. Injection fluids include silicon oil and cross-linked gel. A model was built to simulate the scenario that a hydraulic fracture interacts with a natural fracture. When the hydraulic fracture and the natural fracture interface intersect, the model examines whether the fracture initiation criterion is met for the intact side of the media. The hydraulic fracture can then either step over the natural fracture and propagate into the intact solid or stop at the intersection edge. In this paper, we first introduce the experimental setup and the mechanisms of the fracture propagation across discontinuities. Based on the concepts formulated, we introduce the numerical approach and discuss the modeling results against experimental results. From the tests and modeling we conclude that whether a hydraulic fracture can cross-over a natural fracture depends on the confining stress, injection fluid properties, fracture initiation location and interface coupling of the natural fracture.
The test is to create hydraulic fracture in rock samples via borehole pressurization. Previously, fracturing tests were performed on intact samples by van Dam [1] and Lhomme [2]. Our samples have natural fracture transverse to the borehole. The objective of this test is to characterize the interaction between the hydraulic fracture and a natural fracture. In sample preparation, a 300mm rock cube (nonsaturated) is cleaved into layers by pushing steel slits into the block. When the layers are put back together, fractures are formed in between. Such a fracture is referred to as the natural fracture in our study to differentiate from the hydraulic fracture. Fig 1 shows the schemes of the cleavage that creates one and two natural fractures, respectively. Before or after the cleavage, a 20mm diameter borehole is drilled through the block such that it intersects the natural fracture. The fracturing test is demonstrated in Fig 2, for the dual and triple layered blocks. The hydraulic fracture is initiated by pressurizing a section of borehole by fluid injection. As shown, the wellbore section on one side of the natural fracture is often protected by steel casings. The purpose of such configuration is to test under what condition the hydraulic fracture can cross-over the natural fracture. Table 1 lists all the rock type and injection fluid combinations.
Both cleavage and injection tests are performed within a tri-axial set-up described by Weijers [6]. The left column of Fig 3 shows the photos of the set-up.
