A Shell subsea tieback in GoM has experienced numerous production issues such as wax, asphaltenes, scale, emulsions and sand production. In addition to various flow assurance issues, slugging was also observed after well stimulation jobs. The observed slugging was characterized by high pressure fluctuations and significant water cut variations which lead to platform trips. The slugging combined with other flow assurance issues made the slugging behavior difficult to distinguish and understand from the multiphase flow perspective and to mitigate this phenomenon. The primary purpose of the study was to identify the root cause of the observed slugging patterns via transient multiphase flow simulations. Several mechanisms which could provide reasonable explanation for the slugging behavior have been proposed and investigated. In particular, the mechanisms of riser-induced severe slugging, terrain-induced slugging, growing slugs, and unstable oscillating flow were studied. Other possible explanations related to flowline topography, organic solid deposition and emulsion/sludge have also been investigated. Comparisons between the simulation results and the field data allowed the validation of various slugging mechanisms, identify the most probable root cause of the slugging and recommend mitigation strategies.
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