Design and Optimization of Combined Well-Killing Methods in Ultra-Deep Wells

The geological environment of ultra-deep well drilling is highly complex, leading to more severe kick and overflow situations and posing higher demands on well control technology. Considering the limitations of single well-killing methods in safety and efficiency, this study proposes an optimized combination of different well-killing techniques to effectively control wellbore pressure in ultra-deep drilling. Based on a multiphase flow model of the wellbore during the well-killing process, the pressure variation, gas migration behavior, and pressure-balancing performance were simulated for both single and combined well-killing methods. Under the simulated conditions, the engineer’s method achieved successful well killing but required pre-mixing of heavy drilling fluid and was operationally demanding. The driller’s method required longer time and presented higher pressure risks, while the reverse-circulation method exhibited a greater risk of bottom-hole pressure loss. Among the combined approaches, the “engineer-first then reverse-circulation” method failed, and the “driller-first then reverse-circulation” method was less effective due to excessive dependence on secondary circulation. The “reverse-circulation first then driller” method extended total operation time. Comparative analysis indicates that the “reverse-circulation first then engineer” method achieves the best overall performance, effectively returning most invading gas to the formation, maintaining lower casing pressure, and shortening the well-killing duration. This study can provides theoretical support for solving complex downhole problems such as gas invasion and overflow in ultra-deep wells.