A Comprehensive Analysis of Fracture Initiation and Propagation in Sandstones based on Micro-Level Observation and Digital Imaging Correlation

摘要

As the oil and gas industries have developed, the word "fracture" has come to represent a familiar subject to petroleum engineers. However, fracturing mechanisms are still not clear though a large amount of literature has addressed the subject matter. In this paper, a new comprehensive experimental method is proposed to analyze fracturing mechanisms through a micro-level observation to a macro-level explanation. To assess the tensile resistance of rock, the Brazilian testing method was applied to disc-shaped specimens of two kinds of sandstones. While loading the test specimens using a material testing system (MTS), a high-speed photography system collected images for the digital imaging correlation (DIC) and calculation of the displacements. After the specimen failed and was split into different parts, the cuttings were analyzed using a scanning electron microscope (SEM) and electron-dispersive system (EDS). The micro-level observation of the grain size, the grain shape, and the mineral analysis proved meaningful for the mechanical testing validation, and the DIC technique proved to be a powerful analysis tool, especially showing the sequence of fracture initiation, propagation, and termination. Finally, we identified two kinds of fracturing mechanisms: (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. The specimens in water-saturated condition showed a higher frequency of the center-initiating fractures while the mud-filtrated condition contributed to the fracturing initiation from the loading points, but the main direction of fracturing propagation was still along the diametric loading plane, except for some deviations near the center and near the loading points. 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 on the 3-D effect on a standard 2-D Brazilian test.