Recently, with the ever-increasing development of offshore fields, the use of long risers fed by lateral flow lines has significantly increased. This type of configuration exposes the production system to severe slugging, particularly with decline in reservoir pressure and consequently flow rates. Severe slugging can cause huge pressure fluctuations and may result in decreased recovery and separator flooding. This study is an experimental and theoretical effort to analyze how the application of surfactants can suppress or eliminate severe slugging.
The experimental study is performed in a 2-in ID facility, consisting of a 64-ft flow line with 1° downward inclination, followed by a 41-ft riser. Water and compressed air are the liquid and gas phases, and an anionic surfactant is applied continuously. First, effects of surfactant delivery location are investigated by injecting at three points, namely, riser base and flowline inlet without or with a static mixer, used as an external source of agitation. Pressure and holdup fluctuations with time, slugging frequency and surfactant efficiency are monitored. Moreover, a high-speed camera is used to monitor blowout stage and analyze gas front thickness and velocity. Pressure fluctuations and surfactant efficiency are experimentally investigated by changing the operating conditions. Tests include without and with surfactant at different liquid and gas flow rates.
Firstly, the experiments are conducted with lowest superficial gas velocity (vSg) and highest superficial liquid velocity (vSL) values. At these conditions, severe slugging is partially eliminated only with high surfactant concentration and the use of static mixer. The use of surfactant in other locations results in elimination of slug production stage and reduced cycle duration, but negligible changes in pressure profile. Then, a new test matrix is designed to conduct tests at higher vSg and lower vSL values, moving towards the boundary of Bøe's criterion for severe slugging. Under these conditions, severe slugging cycle durations and pressure fluctuations reduce and elimination efficiency of surfactant increases significantly. In addition, severe slugging severity and elimination efficiency are correlated with Boe's criterion coefficient.
This study is an attempt to characterize severe slugging magnitude based on operating conditions, and investigate surfactant efficiency for different delivery locations. Considering the limited number of studies on this subject, the results can provide an experimental and theoretical source to optimize surfactant application in production systems facing severe slugging.