At high gas rates, even small amounts of liquid can have a significant impact on the pressure drop, and this is important to model accurately when designing wet gas transport systems. In order to gain an improved understanding of this matter, several sets of two- and three-phase experiments in low liquid loading flows with high gas rates were conducted in the Large Scale Loop at the SINTEF Multiphase Flow Laboratory. The experiments were conducted in both a near-horizontal 8" pipe, and a vertical 4" pipe, where particular emphasis was put on studying the effect of water cut. The measurements show that the frictional pressure drop is very sensitive to the water cut in these circumstances, even though the estimated liquid content is typically less than 1%. For instance, at 50% water cut, the frictional pressure drop in the near-horizontal 8" pipe was found to be up to 30% higher than in the respective two-phase scenarios. In the vertical 4" experiments the three-phase effects were found to have similar magnitudes. To explain these surprising results, the physical mechanisms that may be responsible for this phenomenon are briefly examined and discussed.
Figure 1 and Figure 2 show examples of the observed behaviour in the 4" vertical pipe, where the frictional pressure drop divided by that measured for two-phase gas-oil is plotted against the water cut and the liquid rate. These experiments and the associated measurements are described in more detail in the following sections.