Abstract: Due to the high temperature, high pressure, high stress and complex formation environment during the drilling of ultra-deep wells, gas invasion is prone to occur. If not discovered in time, the bottom hole pressure will continue to decrease, and gas invasion will intensify, leading to a vicious circle that increases the risk of well control. The key technology of wellbore pressure control lies in the study of complex multiphase flow laws in ultra-deep wells, but the current research is not deep enough. This study focuses on the high-temperature and high-pressure rheological characteristics of the fluids in the ultra-deep wellbore, and establishes a complex mathematical model of the full wellbore flow based on the drift flux model. The model is solved, verified and analyzed using the example of Well ST8 for pure overflow and simultaneous overflow and leakage. Research results show that the calculated standpipe pressure and casing pressure from this model are basically consistent with the field operation data, and the calculation accuracy is high, which can be used to describe the evolution law of complex flow pressure. Under the same working conditions, the pressure drop of the coexisting overflow is higher than that of the overflow condition, and the consequences are more serious. Both density and flow rate have an effect on the bottom hole pressure, in which the density mainly affects the hydrostatic column pressure, and the displacement mainly affects the flow friction.

Key words: high temperature, high pressure, ultra-deep well, complex flow, pressure evolution law