Evolution laws of wellbore annular multiphase flow under coexisting overflow and lost circulation conditions

Abstract Deep and ultra-deep well drilling frequently encounters narrow pressure window formations with multiple pressure systems, leading to the simultaneous occurrence of formation gas influx (overflow) and drilling fluid loss (lost circulation). Existing studies have largely focused on gravity-displacement-type coexistence within the same layer, while the flow characteristics of the scenario of upper overflow and lower lost circulation (Scenario 2) and the scenario of upper lost circulation and lower overflow (Scenario 1) remain poorly understood. In this study, a large-scale visualised experimental device for wellbore gas-liquid two-phase flow under coexisting overflow and lost circulation conditions was designed and constructed based on geometric and Reynolds number similarity criteria. The device has an annular equivalent diameter of 130.9 mm and a total length of 10 m; two distinct coexisting configurations are reproduced by independently adjusting the gas-injection point and the lost-circulation port. Three flow patterns—bubbly, cap-bubble, and churn flow—were identified in the large-diameter annulus. Under Scenario 1, the flow pattern remains essentially unchanged as the lost-circulation rate increases. Under Scenario 2, a clear bubbly-to-cap-bubble transition is triggered once the lost-circulation rate exceeds 5 m 3 /h. Mechanistic analysis reveals that these contrasting behaviours arise from a fundamental difference in loss mode: gas and liquid are discharged simultaneously at an unchanged gas-liquid ratio in Scenario 1, whereas only liquid is lost in Scenario 2, progressively increasing the in-situ gas-liquid ratio. Based on this mechanistic distinction, correction methods for apparent gas and liquid velocities were formulated for each configuration; after correction, the maximum pressure-gradient prediction error was reduced to below 10% and the average error to below 4%. The findings provide experimental evidence and theoretical support for well-control design under coexisting overflow and lost circulation conditions in deep and ultra-deep wells.