X-ray micro-tomography for investigation of meso-structural changes and crack evolution in Longmaxi formation shale during compressive deformation

Shale is a typical heterogeneous geomaterial and investigation of the meso-structural changes and crack evolution is beneficial to shale gas development. Understanding the microscopic failure mechanism is undoubtedly crucial to hydraulic fracturing treatment. In this paper, real-time high-resolution X-ray Computed (micro-) Tomography (X-ray CT) with was used for the first time to deform shale sample experimentally under unconfined axial compression. Organic matter, pyrite, and micro-fractures were volumetrically rendered with a resolution of 11.27 μm, and 2D/3D image investigations enabled us to explore the structure and fracture evolution of the sample due to in-situ compression. This work reveals that the stress-induced deformation of shale is found to be dependent on organic matter compaction, pyrite spatial evolution, and micro-fracture initiation, propagation, and coalescence. The volume of organic matter decreases with the increase of axial stress; and the spatial distribution of pyrite changes at different loading stages, but the effective volume is almost the same. The formation of macroscopic fractures ∼20° oblique to the loading direction was observed. The cracks initiate in the tensile cracks between the bedding plane at the bottom of the sample and shear fractures are composed of tensile cracks connected by an “X” shape. The layered deposited structure and weak cementation between layers are the main factors controlling the failure mechanism.