Abstract: Pressure-driven fracturing technology has been applied to address the challenge of energy replenishment in low-permeability reservoirs. The operating pressure during fracturing-enhanced flooding is a key parameter that influences the water injection capacity of the reservoir. To determine the appropriate operating pressure, a study on the critical pressure for hydraulic fracture extension based on the fracture process zone (FPZ) was conducted. Firstly, a model for assessing the stability of hydraulic fracture extension was established using elastic mechanics and elastic-plastic fracture mechanics, taking into account the influence of the FPZ and wellbore stress concentration on the stress intensity factor at the fracture tip. Secondly, visual experiments on fracturing-enhanced flooding were designed to investigate the evolution of fractures and validate the reliability of the model. Finally, based on the theoretical model and the injection pressure curve, the upper and lower limits of the operating pressure for fracturing-enhanced flooding were determined. The results indicate that there are two modes of fracture extension: steady-state and non-steady-state, which are governed by the driving and resisting forces of fracture extension. Before the breakdown point, the fractures undergo multiple stages of steady-state and non-steady-state extension. The lower limit of the operating pressure is the turning point where the slope of the injection pressure curve decreases, marking the onset of an increase in injection capacity after a period of decline. The upper limit of the operating pressure corresponds to a stress intensity factor at the fracture tip of 0.05 MPa•m^0.5, which is slightly higher than the endpoint of the last stable fracture extension stage before breakdown and represents the maximum injection capacity. This study provides theoretical support for optimizing the operating parameters of fracturing-enhanced flooding.

Key words: fracturing-flooding, stability of fracture extension, digital image correlation (DIC), fracture process zone (FPZ)