A common problem with gas lifted wells is the development, over time, of instabilities in the injection/production behaviour. The question raised is initially that of “probable cause and effect”; the understanding of which is essential to the determination of possible remedial action.
The major causes of unstable behaviour fall into three broad categories:
• Design related - the original design is inappropriate or inflexible.
• Mechanical - damage to, and/or failure of, valves and equipment.
• Dynamic flow behaviour - changes in fluid composition and/or phase changes.
Commonly, the instability incorporates elements from more than one category.
This paper discusses one case in which a horizontal well in the North Sea, which had a gas lift completion designed for operation at a water cut of around 20%, exhibited unstable production after a rapid rise in water cut to approximately 80%.
The paper shows how a new and unique dynamic gas lift simulator was used to reproduce the observed well behaviour, and how the model was then used to recommend remedial action to stabilise production. The impact of these remedial actions is discussed in the context of the overall production management. Finally, the implementation of the recommendations and the subsequent well behaviour are presented.