Microscopic fracture characterization of gas shale via scratch testing

We investigate the fracture properties of organic-rich shale at the microscopic scale by coupling advanced imaging techniques, fracture mechanics and micro-scale mechanical testing methods. We study three shale systems: Toarcian (Paris Basin, France), and Lower and Upper Woodford shale (Oklahoma, US). A material preparation procedure is designed so as to visualize the microstructure. Optical microscopy and scanning electron microscopy reveal a porous granular fabric with the grain size ranging from 30 to 100 μm as well as micron-sized air voids. Microscopic scratch tests are carried out, during which a stylus is pushed across the surface of a material under a prescribed monotonically increasing vertical load. We develop a fracture mechanics approach that takes into account the heterogeneity and anisotropy of gas shale. The microscopic scratch toughness predicted by the scratch fracture model is 2.35–2.98 MPa m, which is two times higher than the macroscopic fracture toughness. A microscopic examination of the fracture surface reveals toughening mechanisms such as particle pull out, crack front roughening and crack bridging. The methodology presented is new and will pave the way toward a mechanistic physics-based understanding of the fracture behavior of gas shale at multiple length scales. In turn, this will accelerate the design of optimum and efficient schemes to extract natural gas from unconventional shale.