Abstract: Deep and ultra-deep oil and gas wells often encounter abnormal pressures during drilling due to their complex wellbore structures and larger borehole sizes, which results in a narrowed safe operating window. When gas intrusion occurs, a gas-liquid two-phase flow forms in the annulus. Conventional well control methods based on standard flow pattern transition theories are prone to exceeding this narrow window, leading to alternating influx and loss, thereby missing the optimal well control timing. To address this issue, this paper develops a numerical simulation method for gas-liquid two-phase flow in large-diameter annuli using the Volume of Fluid (VOF) model, validated against literature data for accuracy. In simulations conducted within a 196.85 mm annulus, four flow patterns—bubbly flow, cap-bubbly flow, slug flow, and churn flow—were identified. A gas-liquid two-phase flow pattern map was created, and criteria for flow pattern transitions were established, revealing the influence of annular size on these transitions. The results indicate that, compared to conventional annuli, the range of bubbly flow expands in larger annuli, with a transitional flow pattern—cap-bubbly flow—occurring between bubbly and slug flows. The boundaries for flow pattern transitions shift to the right to varying degrees. In conventional annuli, bubble coalescence and the formation of Taylor bubbles are more likely, making well control operations more challenging. Consequently, well control parameters designed for large annuli tend to be oversized. In contrast, the well control parameters designed based on the new criteria better accommodate the needs of well control in narrow windows and large borehole sizes, thereby improving the efficiency and safety of well control operations.