Interaction analysis of propagating opening mode fractures with veins using the Discrete Element Method

Abstract Cemented natural fractures (veins) in shale formations generate mechanical heterogeneities which contribute to complex hydraulic fracture network development. Lee et al. (JGR, 2015) experimentally investigated the interaction of propagating tensile fractures with calcite-filled veins in Marcellus shale using Semi-Circular Bend (SCB) tests. In this study, we investigate those experimental results with three-dimensional Discrete Element Method (DEM) modeling to gain further insight in the fracture interaction processes that generate hydraulic fracture complexity. In addition, we quantified the length of fracture diversion distance to show the effect of vein properties, including approach angle, thickness, strength, stiffness, penetration length, on fracture diversion results. The numerical results compared well with the laboratory fracture diversion results. There is a tendency towards fracture diversion as the approach angle becomes smaller and more oblique to the propagating fracture plane. As the stiffness of the vein increased, the microcrack-related damage increased and the bond breakages extended longer to either side of the main fracture path. Samples with thicker veins and increasing penetration length showed a weaker vein response; the induced fracture propagated for a longer distance in veins before kinking back into the rock matrix. In addition, the numerical model confirmed that the opening mode fracture initiated from the notch tip and the intergranular bond failure created microcracks within the vein before the approaching fracture intersected the vein. The failure mode of the bonds between discrete elements in the vein was predominantly tensile mode rather than shear.