Numerical Simulation of Wellhead Back Pressure in Under-balanced Drilling

Based on the application of liquid-phase under-balanced drilling in a vertical well, this paper studies the wellhead back pressure (WBP) on annular gas-liquid two-phase flow characteristics and its impact on the law of gas cut speed. Annular gas-liquid two-phase flow transient calculation model and a model of gas cut during drilling are established in this study. And used to calculate the changing law of WBP under constant and variable bottom hole underbalanced. Results show that a risk of excessive gas cut or WBP exists when uncovering gas reservoir under constant underbalanced. The WBP needed for using variable negative pressure to keep gas cut speed unchanged remains stable after a period of growth. And is mainly affected by gas cut speed, formation permeability, time to apply WBP, and other factors. The simulation result is compared with the WBP value collected on site in real time, show that the changing trend is roughly the same.