Abstract: To address the problem of unclear transition boundary between bubbly flow and slug flow in the process of gas-water mixed injection to displace the "attic oil" in the Tahe Oilfield, the numerical simulation method was used to simulate and analyze the bubbly flow and slug flow in the gas-water two-phase descending flow. The results show that: under the simulation conditions of gas-phase apparent flow rate of 0.01-1.00 m/s, liquid-phase apparent flow rate of 0.03-2.00 m/s, and pipe diameter of 76 mm, the pipeline is mainly characterized by bubbly flow and slug flow; compared with the simulation results, the transition boundary predicted by the Barnea, Kokal, and Xue Yuxing models is small, and that predicted by the Bhagwat and Yijun models is large; with the increase of the gas-phase apparent flow rate, the liquid volume required for the transition from bubbly flow to slug flow gradually increases; under the condition of low liquid volume, the closer to the center of the pipeline, the higher the number of bubbles and the larger the void ratio; with the increase of liquid volume, the volume of individual small bubbles decreases, and the more uniformly the bubbles are distributed throughout the cross-section of the pipeline. Based on the drift model, a new bubbly flow-slug flow transition boundary model was established by considering the bubble group slipping velocity, and 216 sets of literature data validation results show that the accuracy of the new model is 95.37%, which is high. The established bubbly flow-segment plug flow transition boundary model cannot only improve the calculation accuracy of the wellbore pressure and temperature model, but also has a good theoretical significance for the optimization of wellhead parameters, the selection of injection equipment, and the improvement of the efficiency of "attic oil" displacement in the field of Tahe Oilfield.

Key words: bubbly flow, slug flow, transition boundary, gas-water mixed injection, attic oil, fractured-vuggy reservoirs