A B S T R A C T

Hydraulic fracturing and horizontal well are the two key techniques for the development of shale oil/gas. Mechanical heterogeneity and anisotropy of shale strongly influence the effects of fracturing. While numerous studies have focused on the fracture behavior of shale, the microscale fracture mechanisms of shale remain poorly understood. In this study, three representative test areas, which were dominated by a stiff mineral, a blob of organic matter and clay layers, respectively, were selected in a terrestrial shale sample from the Yanchang Formation. Micro cantilever beams were manufactured using the micro fabrication method in each test area, and in situ fracture experiments were performed to investigate crack propagation under a scanning electron microscope. The crack propagation process was characterized at micrometer scale in shale. The crack paths and the load-displacement curves revealed various fracture mechanisms. Crack deflection and crack branching owing to mechanical contrast, toughening in organic matter, and crack bridging of clay layers were observed and discussed. This study provided a novel insight into micro crack behavior in shale and presents a new framework for the investigation of the fracture characteristics of shale. The microscale shale characterization and crack mechanisms can serve as a basis or building blocks for mesoscale modeling of fracturing in shale, and the load-displacement data and fracture behaviors can provide a valuable dataset for validating modeling approaches.

 

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