Hydraulic fracturing characteristics and evaluation of fracturing effectiveness under different anisotropic angles and injection rates: An experimental investigation in absence of confining pressure

Understanding and characterization of fracture propagation are crucial for enhancing the stimulation of low-permeability gas reservoirs. This study investigates the effects of shale laminate anisotropic angles (between the bedding plane and the coring orientation) and injection rates on fracture propagation in the absence of confining pressure. Results show that the variations of breakdown pressure under the uniaxial stress state are significantly different from those under conventional confining pressure: when the anisotropic angle changes from 0° to 90°, the breakdown pressure first increases and then decreases; for the increasing injection rate, there shows an apparent downward trend for the breakdown pressure. Based on macroscopic morphology descriptions, stereomicroscope imaging, and CT scanning, we show that under uniaxial stress, changing the anisotropic angle close to 45° and reducing the injection rate contribute to a more tortuous fracture path and larger stimulated fracture volume. In addition, the intersection between hydraulic fractures and bedding planes is an immediate cause of sinuous fracture morphology. Finally, three dimensionless indicators were employed to quantitatively evaluate the fracture morphology and fracturing effectiveness, which reported that the samples with low injection rate and large anisotropic angle (>45°) could achieve optimal fracturing results. The findings of this study provide important guidance for optimizing the treatment design of hydraulic fracture networks.