The elimination of severe slugging-experiments and modeling

Severe slugging can occur in a pipeline-riser system operating at low liquid and gas rates. The flow of gas into the riser can be blocked by liquid accumulation at the base of the riser. This can cause formation of liquid slugs of a length equal to or longer than the height of the riser. A cyclic process results in which a period of no liquid production into the separator occurs, followed by a period of very high liquid production. This study is an experimental and theoretical investigation of two methods for eliminating this undesirable phenomenon, using choking and gas lift. Choking was found to effectively eliminate or reduce the severity of the slugging. However, the system pressure might increase to some extent. Gas lift can also eliminate severe slugging. While choking reduces the velocities in the riser, gas lift increases the velocities, approaching annular flow. It was found that a relatively large amount of gas was needed before gas injection would completely stabilize the flow through the riser. However, gas injection reduces the slug length and cycle time, causing a more continuous production and a lower system pressure. Theoretical models for the elimination of severe slugging by gas lift and choking have been developed. The models enable the prediction of the flow behavior in the riser. One model is capable of predicting the unstable flow conditions for severe slugging based on a static force balance. The second method is a simplified transient model based on the assumption of a quasi-equilibrium force balance. This model can be used to estimate the characteristics of the flow, such as slug length and cycle time. The models were tested against new severe slugging data acquired in this study. An excellent agreement between the experimental data and the theoretical models was found.