The Influence of Bedding Planes on Tensile Fracture Propagation in Shale and Tight Sandstone

Unconventional oil and gas reservoirs are normally characterized by low porosity and low permeability and need to be hydraulically fractured to produce fracture networks and achieve the potential for economic production. Reservoir rocks are generally characterized by bedding planes, which have a significant impact on fracture propagation. In this study, a double torsion experiment was used to simulate tensile fracture propagation during hydraulic fracturing. The effect of bedding planes on fracture toughness, fracture morphology, and propagation rate in shale and tight sandstone was studied. Micromechanical mechanisms were analyzed through a microindentation experiment. The results show that as the angle between bedding and a pre-cut plane increases, fracture toughness first decreases and then increases, while the subcritical fracture propagation rate first increases and then decreases. The inflection point for this behavior occurs at an angle of 30 degrees between the bedding and pre-cut plane. The shape of the tensile fractures is mainly influenced by the micromechanical properties of the adjacent bedding plane. When the ratio of elastic modulus between adjacent beds is 0.67, fractures propagate along the bedding plane, while at a ratio of 0.79, fractures extend through the bedding plane. These findings indicate that the angle between bedding and artificial planes as well as the difference in micromechanical properties of adjacent bedding planes are important to judge whether hydraulic fractures can penetrate or spread along a bedding plane.

Automated summary

In this study, a double torsion experiment was conducted to simulate the propagation of tensile fractures during hydraulic fracturing. The influence of bedding planes on fracture toughness, morphology, and propagation rate in shale and tight sandstone was investigated. The results indicate that the angle between bedding and pre-cut plane, as well as the difference in micromechanical properties of adjacent bedding planes, are crucial in determining whether hydraulic fractures can penetrate or spread along a bedding plane.