A new calculation model for the stress field of hydraulic fracture propagation at the formation interface

Reservoirs with different formation interfaces are becoming increasingly common in the exploitation of unconventional oil and gas reservoirs, such as shale oil and gas. Therefore, it is essential to calculate the stress field of fracture propagation to understand the mechanism of propagating hydraulic fractures near formation interfaces. This study adopts the superposition principle of same-type fractures and considers the model propagating hydraulic fractures at interfaces as the superposition of three issues: actuating the fluid pressure inside the fracture without far-field stress, analysis with far-field stress and without fractures, and calculating the self-balancing surface force effect on the surface of fractures without far-field stress. Then, the study establishes the stress field calculation model for propagating hydraulic fractures at the formation interfaces, which considers different rock properties on both sides of the formation interfaces, as well as the influence of the dip angle based on the complex variable function method and fracture mechanics theory. Finally, this paper proposes corresponding numerical methods. It is determined by analyzing different stress concentration which occurs around the fractures and stress singularity which occurs at the hydraulic fracture tips, analyzing the plastic zone at hydraulic fracture tips which propagates contacts with formation interfaces; the maximum compressive stress and normal stress change gradually from tensile stress to compressive stress as the distance between the interface and the hydraulic fracture tip increases, and the exhibiting stress reverses on the formation interfaces.