This paper is intended to investigate the occurrence and characteristics of density-wave instability in gas-lift wells when critical gas injection is attained at the gas-lift valve. Based on a simplified gas-lift system, both theoretical analysis and numerical simulations are performed so that crosscheck can be made in order to gain confidence about the results. The study demonstrates that it is possible for density-wave instability to occur in those gas-lift wells producing from depleted reservoirs. The theoretical analysis and numerical simulations give the similar parametric trend in characterizing the instability. Simulation also shows that the average production could be significantly reduced due to the instability compared with the steady-state prediction. An attempt of using feedback control to stabilize the simplified gas-lift system is also tested within the simulator, from which promising results are obtained in both stabilization and increasing production. The results of this investigation add new knowledge to gas-lift instability fundamentals and can help in diagnosing and remedying unstable gas-lift problems.
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