Abstract

The goal of this study is to gain more insight into crack initiation, propagation and coalescence patterns in laboratory tested brittle rock by PFC2D modeling. In the first part, a benchmark study is performed on samples containing pre-existing single flaw under compressive loading. Tensile wing cracks are exclusively the only primary cracks that initiate from a single pre-existing opening or closed flaw. Secondary cracks mostly form through coalescence of tensile micro-cracks. The second part of the study investigates fracture propagation from a fluid pressurized pre-existing opening by using a hydro-mechanically coupled fluid flow model under isotropic and anisotropic compressive stress states. The main mechanism of hydraulically induced fracture is tensile failure. In contrast to mechanically driven fracture propagation, the models involving fluid injection exhibited different fracture propagation pattern: no secondary cracks are observed in the pressurized flaw case and the developed wing cracks can reach the boundaries of the specimen.

1 Introduction

Cracking mechanisms have been extensively studied in brittle rock and rock-like materials under compressive loading through experimental (e.g. Bobet & Einstein 1998a and Wong & Einstein 2009) and numerical approaches (e.g. Bobet & Einstein 1998b and Zhang & Wong 2012). Wong & Einstein (2009) give an in-depth review on cracking behavior in specimens containing a single flaw under uniaxial compression. They conclude that in the previous studies in most of the tested specimens tensile wing cracks were observed first, and shear cracks were never observed to be the first fractures. They also note that secondary cracks are not necessarily shear cracks in nature, in contrast to fracture mechanics literature (e.g. Bobet & Einstein 1998a and 1998b).

Similar to the case of compressive loading, Shimizu et al. (2011) emphasize that the mechanism of hydraulic fracture growth needs more clarification. Whereas most of the earlier studies investigated hydraulic fracturing from a circular borehole (e.g. Al-Busaidi et al. 2005 and Shimizu et al. 2011), only Zhao et al. (2012) have investigated wing crack initiation and development from an inclined pre-existing flaw using finite element modeling (FEM).

Here we use discrete element method based bonded particle model (Particle Flow Code 2D) to simulate and analyze fracture initiation and propagation mechanism in granitic rock under compressive loading and hydraulic fracturing. This study aims to compare the crack growth path under these two different loading conditions: mechanical and hydraulic driven fracture propagation.

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