Isotropic versus Anisotropic Stress Field Effects on Hydraulic Fracture Mechanisms in Opalinus Shale

Although hydraulic fracturing has been widely used for decades, and the technology to implement and interpret the induced fractures has been continuously evolving, many aspects are still not understood. Specifically, this includes hydraulic fracture initiation and propagation mechanisms and the effect of stress state and rock fabric. The objective of this study was to determine the differences between hydraulic fracturing under isotropic and anisotropic stress conditions. The rock used in this study is Opalinus Shale prepared into prismatic specimens with a pre-existing artificial fracture (flaw) in the middle. Different external biaxial stresses are applied to simulate in-situ stress conditions followed by hydraulic pressurization of the flaw until failure. Internal flaw pressure is measured throughout the pressurization and fracturing process. High-speed and high-resolution cameras are used for visual analysis. Two experiments are presented, discussed in detail and compared: 1- a specimen with a bedding plane orientation of 30° relative to horizontal is subjected to a vertical stress of 3 MPa and a lateral stress of 1 MPa (anisotropic stress). 2- a specimen with the same bedding plane orientation of 30° is subjected to biaxial isotropic stresses of 2 MPa (isotropic stress). The results show that the combination of rock fabric and stress state affect the initiation and propagation of hydraulic fractures in shale. This adds to fundamental knowledge on how fractures behave and may provide insight into strategic hydraulic fracture treatments for field applications.