Natural oil-gas-bearing rock formation generally contains a large number of discontinuities, which have a large influence on the cracking process inside the rock under high water pressure condition. Hence, a good understanding of the propagation and coalescence of fluid-driven cracks is important to improve the oil and gas exploitation efficiency. This study numerically investigates the fluid-driven crack propagation process in rock specimen possessing two pre-existing flaws using a fluid coupled discrete element method. The micro-parameters are first calibrated against the mechanical properties of the Lac du Bonnet granite. The numerical specimen models are generated by installing two flaws with different ligament angles, ligament length, and flaw angles. The injection test with a constant rate is then conducted to study the propagation and coalescence patterns in these pre-cracked numerical models. The numerical results show that a relatively large ligament angle is better to accelerate the crack coalescence in the specimen with two paralleled flaws. Numerical model possessing two pre-existing flaws with a small ligament length is inclined to traverse the ligament area. Overall, the results in this study reveal that the pre-existing flaws inside the rock have a significant effect on the fluid-driven cracking process and much attention should be paid to the crack propagation behavior when many discontinuities are associated in the oil-gas-bearing rock formation.
Keywords: Hydraulic fracture, Pre-existing flaws, Discrete element method, Crack propagation and coalescence, Grain-based modeling approach