Cracking properties of shale influenced by bedding layers and a pre-existing slot

Abstract

The key to increasing shale gas production is to construct fracture networks in shale reservoir to provide channels for shale gas. Understanding the cracking characteristics of shale is necessary for oil and gas exploitation engineering. Given this, uniaxial compression tests were conducted on Longmaxi shale in China to study the mechanical properties and cracking behaviors affected by bedding layers and pre-existing slot. Sandstone specimens with different pre-existing slot angles were also tested as a comparison. A mechanical-optical-acoustical comprehensive data acquisition system consisting of a rigid hydraulic machine, high-speed industrial camera and acoustic emission acquisition instrument was established to monitor the cracking behaviors in real time. The results show that the cracking behaviors of shale specimens are quite different from sandstone specimens in the uniaxial compression tests. Crack initiation is predominantly controlled by the pre-existing slot and is also affected by bedding layers. Crack propagation is mainly controlled by bedding layers and stress field distribution. When the bedding layers are vertical, the cracks are most likely to propagate along the direction of the bedding and tensile cracks are observed. When the bedding is 30°, the shale specimens are most likely to be controlled by the bedding layers, resulting in shear slip failure along the bedding layers. The experimental results contribute to the understanding of cracking properties in layered anisotropic materials.