Observations of Fractures Induced by Hydraulic Fracturing in Anisotropic Granite

To investigate how the viscosity of the fracturing fluid affects fracture propagation, hydraulic fracturing experiments using three fluids with different viscosities (supercritical CO₂, water, and viscous oil) under the true tri-axial condition were conducted on anisotropic granite specimens, and then the induced fractures were microscopically observed via a fluorescent method. Fractures induced by hydraulic fracturing are considerably tortuous from a microscopic view. A higher viscosity creates a smoother fracture pattern. The tortuosity, which is defined as the total fracture length along a pathway divided by the direct length of the two ends of a fracture, ranges from 1.05 to 1.13, demonstrating that the viscosity of fracturing fluid influences the fracture propagation pattern due to the different pathways of fracture propagation. In addition, hydraulic fracturing can induce many derivative pathways around the main fracture. Hydraulic fracturing with a lower viscosity fluid forms a more complex fracture network in rocks; the fracture induced by supercritical CO₂ has the most branches along the main fracture. From these observations, fracture propagation by hydraulic fracturing sometimes develops by the shear fracture mode. This shear fracturing is often observed for a low-viscosity supercritical CO₂ injection, which agrees with our results from AE monitoring and waveform analysis.