Study on the mechanism of fracture propagation and deflection near wellbore in ultra-deep high stress difference reservoir

With the exploration and development of oil and gas into the ultra-deep reservoir, hydraulic fracture propagation under the condition of high stress difference is prone to occur large curvature deflection, leading to wellhead overpressure, sand plugging and other problems occur frequently. It is of great significance to clarify the mechanism and main control factors of hydraulic fracture propagation and deflection near wellbore in ultra-deep and high stress difference reservoir for safe and efficient development. Under the constraint of the continuity framework of classical thermodynamics, the sharp fracture on the discrete interface is smoothly described as a continuous damage dispersion fracture, and the Lagrange Energy Functional is constructed based on Griffith energy balance relation and fracture variational principle, and then the phase field hydraulic fracturing model of perforated well in anisotropic reservoir is established based on the principle of energy minimization. The validity of the phase-field model presented in this paper is verified by comparing with the classical Griffith crack opening profile equation. It is found that the hydraulic fracture starts to crack along an approximate straight line between perforation and maximum horizontal principal stress, and then deflects to the maximum horizontal principal stress direction. The specific direction of crack initiation is affected by in-situ stress difference, displacement and perforation angle. The increase of the in-situ stress difference will promote the hydraulic fracture deflection propagation near the wellbore, make the deflection angle increase and the deflection radius decrease; increasing the displacement can weaken the hydraulic fracture deflection, and making the deflection angle decrease and the deflection radius increase; with the increase of perforation angle, the angle between perforation and the maximum horizontal principal stress increases, which will aggravate the deflection degree of crack propagation so that the flow friction of fracturing fluid increases and the risk of sand plugging increases; the anisotropy characteristics of the reservoir can also significantly affect the deflection and propagation process of hydraulic fractures. In the model, the critical energy release rate in different directions is taken as the anisotropic parameter of fracture resistance. The results show that fractures tend to propagate in the direction of low resistance. The stronger the anisotropy of fracture resistance, the greater the deflection degree of hydraulic fractures. The anisotropic characteristics of reservoir fractures significantly affect the turning behavior of fractures. The phase-field hydraulic fracturing model in this paper provides a convenient method to study the propagation and steering behavior of hydraulic fractures without any fracture criteria, which is helpful to improve the understanding of near-well fracture steering in ultra-deep and high stress difference reservoirs, help to understand the fracture mechanism and fracture deflection behavior under different geological environments and fracturing conditions, and provide reference and suggestions for fracturing technology design and perforation scheme optimization.