Fluid-driven micro-cracking behaviour of crystalline rock using a coupled hydro-grain-based discrete element method

Abstract

The grain-scale heterogeneity of rock has been found to greatly affect the cracking behaviour in mechanical tests. However, the heterogeneity has been insufficiently considered in hydraulic fracturing. To fill the gap, this paper proposes a coupled hydro-grain-based discrete element model and explores the fluid-driven micro-cracking of crystalline rock. The micro-heterogeneity of the rock is represented by a grain-based model. Three laboratory-scale fracturing cases over granite under different in-situ stresses are simulated. First, typical micro behaviour and hydro/hydro-mechanical responses are presented and interpreted. Next, sensitivity analysis of in-situ stress conditions is carried out. It is found that hydraulic fracturing of crystalline rock involves many unique behaviours on grain-scale such as non-symmetrical propagation, “zipper mode” initiation time order, generations of isolated and branching cracks, and re-closure of the cracks earlier initiated. Cracks can be induced in mineral grains and along the grain interfaces. The cracking mode is dominated by tension failure, with a small proportion in shear (between 3.5% and 1.5%). The interplays among orientation, aperture and number of different types of crack show that: 1. The inclination of cracks to the direction of maximum confining stress may occupy the entire range from 0° to 180°; 2. The aperture of cracks decreases statistically with their orientations turning to the direction of the minimum confining stress (σ3); 3. Increases in σ3 suppress the generation of total cracks and tend to force cracks to propagate in mineral grains; 4. A small in-situ stress difference results in fewer branching and isolated cracks. In this paper, evolutions of injection pressure and rock deformation during hydraulic fracturing are also discussed.