Impact of laminations and natural fractures on rock failure in Brazilian experiments: A case study on Green River and Niobrara formations

Natural fractures and laminations are common features in many unconventional resources which require hydraulic fracturing for economic production. To better understand the effect of such features on rock failure, we have conducted indirect tensile or Brazilian testing on several rock samples from Green River and Niobrara formations. Brazilian experiments were conducted at various orientations of laminations or natural fractures with respect to the loading direction. In contrast to isotropic rocks, we observed that rock failure can occur away from the vertical central line of rock parallel to lamination or natural fracture when the approaching angle between the lamination and loading direction is less than 30° in the tested samples. It was also determined that the tensile strength parallel to laminations or calcite-filled fractures is lower than the tensile strength perpendicular to lamination and/or the natural fractures; for instance the tensile strength of Niobara calcite-filled fractures were almost one third of the rock matrix.