The influence of bedding interface strength on the vertical propagation of hydraulic fractures

Bedding interfaces in unconventional oil and gas may influence the vertical propagation path of hydraulic fractures. In this article, the cohesive elements were used to describe the tensile and shear damage of bedding interfaces. The vertical propagation law of hydraulic fractures under different stress, bedding interface tensile strength, and shear strength conditions was calculated. The simulation result revealed three types of hydraulic fractures: “H”-shaped, “fishbone”-shaped, and “I”-shaped fractures. The large vertical stress can cause difficulties in damaging bedding interfaces. When the vertical stress difference lies between 0 and 5 MPa, complex fishbone-shaped fractures can easily form. However, when the vertical stress exceeds 5 MPa, “I”-shaped fractures are more likely to occur. In case the vertical stress difference is less than 0 MPa, hydraulic fractures may encounter obstacles in crossing the bedding interface, resulting in the formation of “H”-shaped fractures. A larger tensile strength of the bedding interface will promote hydraulic fractures to pass through the bedding interface and make it easier to form “I”-shaped fractures, promoting the growth of fracture height, which promotes fracture height growth. On the other hand, a low shear strength in the bedding interface can cause shear slip, resulting in the formation of “H”-shaped fractures, which inhibits the growth of fracture height. Tensile shear mixed damage typically happens at the bedding interface. This type of damage can restrict the vertical propagation of hydraulic fractures, but it can also make the fractures more complicated.