Investigation of the effect of initial structure and loading condition on the deformation, strength, and failure characteristics of continental shale

The study of the mechanical characteristics of lamellar continental shale is of great theoretical and application value for unconventional shale oil production. In this study, four cuboid continental shale specimens were subjected to true triaxial compression tests to investigate their deformation, strength, and failure characteristics. The initial structures were analysed at three observational scales using optical photography, three-dimensional microscopic and thin section observations. Specimens were tested under different intermediate stresses and the force–displacement data in three directions were monitored. The failure characteristics were studied using optical photography and non-destructive X-ray computed tomography (CT). It was found that the continental shale specimens had high heterogeneity and were composed of a large number of natural fractures and lamina with uneven thicknesses, and the macroscopic mineral bands were of two main types: those mixed with clay minerals and those mixed with quartz, feldspar and mica minerals. Under four applied stress conditions, the shale deformation stiffness increases with the increasing horizontal stress difference, and the peak strength of continental shale conforms to the Mogi-Coulomb criterion. The failure mode of continental shale interacts with natural horizontal fractures and new inclined/vertical fractures which develop along the direction of intermediate principal stress. With increasing horizontal stress differences, the inclined shear cracks are flatter, the horizontal crack and the connection between horizontal cracks with oblique/vertical cracks are more obvious. The initial structural characteristics and the stress conditions jointly affect the stress–strain curve response with stress fluctuation points and crack characteristics in "bricklaying" form, producing a stress–structure-controlled failure mode. These conclusions have value for engineering applications and provide theoretical guidance for the design of continental shale oil recovery schemes.