An experimental study of vertical two-phase flow was conducted at the SINTEF Multiphase Laboratory using a 55 meter long 4" pipe at system pressures of 20, 45 and 70 bara. One of the main objectives of the campaign was to identify regions with multiple solutions, where two flow regimes can coexist in the pipe for extended time periods: one with a high holdup and one with a very low holdup. This is a critical matter for gas wells, because the discontinuous jump from low to high holdup marks the beginning of the end of the well's life (this phenomenon is sometimes referred to as "liquid loading"). The experiments confirmed that multiple solutions do exist in vertical two-phase flows. Furthermore, it was found that the associated transition between high and low holdup is very sensitive to the gas density. The data analysis showed that the interfacial shear stress becomes very high near the transition, and none of the known models for interfacial friction were able to reproduce that. To improve the situation, a new interfacial friction model was derived using the new data, and implemented in LedaFlow. The new model improved the predictions of the high/low holdup transition significantly in both 4" and 8" pipes, indicating that it scales well with the pipe diameter. It is believed that with this new model, it will be possible to predict the longevity of wells much more accurately than before.
In a natural gas production well, some liquid may be present in addition to the dominating gas phase. This liquid may be water or hydrocarbons, coming from the reservoir or appearing due to condensation in the well. This small liquid fraction must be carried upwards and out of the well by the gas flow, otherwise it accumulates and increases the gravitational component of the pressure drop in the well, a phenomenon referred to as liquid loading (see for example [1]). Ultimately, the well may " drown" as a result of liquid loading if remedial procedures are not initiated. Since the reservoir pressure decreases over the lifetime of a well, this problem will occur sooner or later, the only question is when. Methods exist to produce from a well after this has happened, but there are substantial costs and disadvantages associated with such procedures, and the production will usually continue at a lower rate [2].
