A comprehensive analysis of fracture initiation and propagation in sandstones based on micro-level observation and digital imaging correlation

Abstract

Along with the development of the oil & gas industry, the word "fracture" is now a quite familiar subject to well stability and integrity over the drilling process. However, the fracturing mechanisms are still less convincing though a large amount of literature have addressed the subject matter. In this paper, a new comprehensive experimental method is proposed herein to analyze the fracturing mechanisms through a micro-level observation to a macro explanation.To assess the tensile resistance of rock, Brazilian Test was applied to disc-shaped specimens of isotropic sandstones. Meanwhile, a high-speed photography system was keeping snapping images for the digital imaging correlation (DIC) to get a dynamic fracturing process. After the specimen failed and was split, the cuttings were analyzed using scanning electron microscope (SEM) and electron-dispersive system (EDS) for a micro-level observation.Two kinds of fracturing mechanisms were identified as: (1) grain fracturing, and (2) pore-fillings fracturing. Three basic fracture modes (Mode I/II/III) can also be distinguished through (1) the grain size, and (2) the grain shape and uniformity using a new parameter named angularity factor. The DIC technique proved to be a powerful analysis tool, especially showing the sequence of fracture initiation, propagation and termination. Though the cracks can be initiated from the center or near the loading points, the main direction of fracturing propagation was still along the diametric loading plane. In addition, a displacement along the thickness was also identified through a 3-D investigation, which should be of interest due to the scarcity of relevant literature in 3-D effect on standard 2-D Brazilian Test.