Hydraulic Fracture of Opalinus Shale under Uniaxial Stress: Experiment Design and Preliminary Results

摘要

The interaction of hydraulically induced fractures with pre-existing fractures in shales is of interest to the petroleum industry. Laboratory experiments can help to understand the fracture initiation, propagation, and coalescence behavior. A fundamental first step is to investigate the fracture interaction of a pressurized flaw (artificial crack) with a non-pressurized flaw where the specimens are subjected to a constant uniaxial stress. This paper describes the hydraulic fracture experiment design, which allows us to pressurize an individual flaw, monitor the internal flaw pressure throughout pressurization and fracturing, and visually capture the underlying fracture mechanisms. The experiments are performed on prismatic Opalinus Shale specimens with two pre-existing flaws of various geometries. After subjecting the specimens to a constant uniaxial stress, one of the flaws is pressurized until a hydraulic fracture initiates and propagates. The interaction of the hydraulic fracture with the non-pressurized flaw is observed. Three flaw geometries are investigated in this study: a single vertical flaw, double flaws with a step offset of 30°, and double flaws with a step offset of 60°. The first geometry was tested as a proof of concept for the experimental setup and showed the basic fracture initiation and propagation behavior. The second and third geometries capture the interaction of the hydraulic fracture with the non-pressurized flaw. Although there is only a 30° difference in the stepped angles between the two flaws, the fracture behavior is drastically different.