Influence of Natural Fractures and Laminae on Fracture Propagation and Failure Mode of Continental Shale

Natural fractures and laminae are well-developed in continental shale, which greatly affects the fracture propagation and failure mode. Based on the natural fractures and laminae developed in the outcrops of Triassic continental shale from the southern Ordos Basin, China, four different types of shale models are constructed in this research. The CASRock software V1.0 is utilized to conduct numerical simulations to investigate the influence of natural fractures and soft-to-hard laminae on the mechanical behavior of continental shale. The results demonstrate that the uniaxial compressive strength of shale models can improve by up to 34.48% when soft-to-hard laminae are present, but it can drop by up to 18.97% when weak interfaces are present. New fractures are consistently initiated at the ends of natural fractures, with various propagation patterns in different laminae. Fractures in soft laminae usually propagate in an oblique path at an angle β ≈ 20°-30° relative to the direction of compressive stress, manifesting as shear fractures. Fractures in medium-to-hard laminae tend to propagate parallel to compressive stress, primarily featuring tensile fractures. The ultimate fracture morphology becomes more complex as soft, medium, and hard laminae and weak interfaces occur successively. It changes from a nearly linear fracture to an echelon pattern with more secondary fractures and finally a network shape, with a total fracture area increase of up to 270.12%. This study reveals the combined effect of natural fractures, soft-to-hard laminae, and weak interfaces on the fracture propagation and failure model of continental shale, providing support for fracturing optimization based on shale's authentic structure characteristics.