Mechanical Property Measurements and Fracture Propagation Analysis of Longmaxi Shale by Micro-CT Uniaxial Compression

Abstract

The mechanical properties and fracture propagation of Longmaxi shale loading under uniaxial compression were measured using eight cylindrical shale specimens (4 mm in diameter and 8 mm in height), with the bedding plane oriented at 0° and 90° to the axial loading direction, respectively, by micro computed tomography (micro-CT). Based on the reconstructed three-dimensional (3-D) CT images of cracks, different stages of the crack growth process in the 0° and 90° orientation specimen were revealed. The initial crack generally occurred at relatively smaller loading force in the 0° bedding direction specimen, mainly in the form of tensile splitting along weak bedding planes. Shear sliding fractures were dominant in the specimens oriented at 90°, with a small number of parallel cracks occurring on the bedding plane. The average thickness and volume of cracks in the 90° specimen is higher than those for the specimen oriented at 0°. The geometrical characterization of fractures segmented from CT scan binary images shows that a specific surface area correlates with tortuosity at the different load stages of each specimen. The 3-D box-counting dimension (BCD) calculations can accurately reflect crack evolution law in the shale. The results indicate that the cracks have a more complex pattern and rough surface at an orientation of 90°, due to crossed secondary cracks and shear failure.

Keywords

• shale
• fracture propagation
• layer orientation
• micro-CT
• uniaxial compression