Experimental study on the critical gas velocity of liquid-loading onset in an inclined coiled tube

Coiled tubes have been widely used in gas fields with low permeability because of their small flowing section area and low critical gas rate of liquid loading. An accurate prediction of the critical gas rate in coiled tubes is the basis to optimize the production rate of a gas well. In the current study, an experiment was performed using air and water in pipes with inner diameters of 30 mm and 40 mm at an inclination angle of 15-76° from the horizontal. The pressure gradient at a gas rate of 10-160 m3/d and a liquid rate of 1-10 m3/d was measured. Meanwhile, 72 date points of the critical gas velocity of liquid loading were obtained based on observation and pressure gradient fluctuation characteristics. The effects of the inclination angle, pipe inner diameter and liquid velocity on the critical gas velocity were studied. The correlation between the pressure gradient at the liquid-loading condition and the critical gas velocity was also analysed. The Film model (Xiao, 2010) and the Belfroid model were evaluated against these data points. Two correction items were added to the Belfroid model to consider the effect of the pipe inner diameter and liquid velocity based on the experimental data. The performance of the modified model was significantly improved with an average error of 6.58% and an average absolute error of 11.63%.