Imaging the Deformation of Two Intersecting Orthogonal Fractures

Transitioning from an individual fracture to a 3D fracture networks increases the complexity of a fracture system by introducing intersections. Though intersections compose only a tiny fraction of the total void volume of a fracture network, closure or opening of an intersection can radically affect the transport paths though a network. Here, we image the changes in fracture network geometry as the orientation of a network is rotated relative to an applied stress field. 3D printing was used to print 4 blocks (~ 15 x 14.5 x 20 mm3) to create fracture networks with 2 orthogonal intersecting fractures with either a + or x orientation relative to an applied vertical load (50N-2kN, 0.1 mm/minute). The sample was unconfined in one horizontal direction. In the other, a custom-designed 3D-printed frame was used to maintain a constant force (~320 N). 2D X-ray radiograms were recorded during loading-unloading cycles. Prior to 3D X-ray scanning, one loading-unloading cycle was performed. 3D scans were acquired at 50N, 2kN and again at 50N. From X-ray tomographic images, differences in the void geometry of + and x networks were observed. Using the ratio of the high-to-low stress cross-sectional area, the horizontal fracture in the + closed with increasing stress (0.74), the vertical fracture opened (1.15) and the intersection remained relatively open and unchanged (0.97). Rotating the + to the x orientation resulted in shearing that caused a relative displacement of ~1.4 mm along the intersection, closing regions along the intersection. The / fracture of the x network closed more than the \ fracture. When the x network was unloaded, the fracture geometry was not recovered and the apertures were similar to those observed at high stress. These observations demonstrate that a simple rotation of a given fracture network topology relative to an applied stress field impacts the void geometry along the intersection and the fractures. Knowledge of the expected response of a fracture network to perturbations in stress is crucial for sustainable management of geothermal or CO2 sequestration sites. Acknowledgment: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Geosciences Research Program under Award Number (DE-FG02-09ER16022).