Creation of Complex Hydraulic Fractures Due to Macroscopic Rock Heterogeneity

Hydraulic fracture complexity in unconventional formations, such as shales, has been predominantly associated with the interaction of hydraulic fractures with pre-existing natural fractures. In this study, we demonstrate a novel experimental evidence that shows complex fractures can be induced in intact specimens that are mechanically heterogeneous in the absence of any pre-existing fractures. Synthetic materials are used to cast sheet-like, porous test specimens that have strongly-bonded layers with different mechanical properties. The layered specimen is placed between two thick, transparent plates and constant, anisotropic far-field stresses are applied to the specimen. Fracturing fluid is injected in the center of the specimen, and the induced fracture trajectories are captured with high resolution digital images and subsequent image processing. First, we show that a bi-wing, planar fracture is induced in the layered specimen along the maximum far-field stress direction when the applied differential stress is relatively high. However, when the applied differential stress is relatively low, the induced fractures become complex with multiple wings and nonplanar trajectories. Fracture complexity can also arise under relatively high differential stress when the hydraulic fracture is induced in a thin layer bounded by thicker and harder layers. When the applied differential stress is zero or extremely low, the induced hydraulic fractures become notably less complex, and the fracture propagation direction becomes controlled by the specimen heterogeneity. Recent field evidence by coring through a stimulated rock volume (SRV) in the Eagle Ford Shale showed the formation of complex fractures despite the sparseness of pre-existing fractures in the cored sections of the SRV. Using well-controlled laboratory experiments, our results prove that rock heterogeneity is a plausible and important mechanism for generating complex fractures.