An investigation of the evolution of the internal structures and failure modes of Longmaxi shale using novel X-ray microscopy

An accurate description of the evolution of the internal structure of shale during loading and fracturing is important to understand the failure mechanisms that are related to shale-gas migration. The aim of this work was to investigate and characterize the void evolution and failure of Longmaxi shale quantitatively under uniaxial tests, and analyze the relationship between the distribution of the meso-scale (voxel-scale) minerals and the failure mode. Novel X-ray microscopy combined with an in-situ microtest device was used and three groups of shale specimens were tested under uniaxial conditions with different scanning stresses. Some three-dimensional stereograms of different loading forces and an entire force–displacement curve were obtained for each in-situ test. Based on the results, an evolution of the void distribution was characterized and divided into four stages: weakened damage, linear, damage evolution and stable development, and accelerated damage development. The degree of development of the final cracks was distinguished by the crack volume, equivalent aperture width and connectivity rate. The structure evolution and failure mode of shale were described quantitatively by the void changes and the crack characteristics. Three phenomena of intergranular fracture, transgranular fracture and arrest cracks were observed at a meso-scale to explain the effect of mineral distribution on the crack pattern. These quantitative results can provide a guide for the design in shale fracturing engineering and some references for numerical analysis.