Plastic bag model of the artificial gas lift system for slug flow analysis

• The “plastic bag” model of the gas-lift system is proposed for analysis of casing-heading instability.
• The “plastic bag” model provides simultaneous discretization of space and time for solving partial differential equations.
• Comparisons among several state-of-the-art models for the gas-lift systems are conducted.

Artificial gas-lift systems are commonly used to enhance the production rate of mature oil wells. However, the technology is subject to casing-heading instability resulting in slug flow occurrence, which may significantly reduce the production rate and impose negative influences on downstream equipment. In order to analyze the casing-heading or slug flow phenomenon, a novel mathematical model for describing the behaviors of the gas-lift system is proposed, which is referred to as the ‘plastic bag model’. The plastic bag model is realized by discretizing the differential equations describing the gas lift dynamics simultaneously with respect to both time and space, and the ‘homogenous assumption’ across the whole tubing is no longer necessary. Therefore, compared with the third-order models and the recently proposed high-order model, the plastic bag model can provide more accurate results in both stable and unstable modes. Moreover, the plastic bag model is much more computationally efficient compared with some other methods of solving nonlinear partial differential equations. Simulations for the plastic bag model are conducted, and comparisons between the plastic bag model and the third-order model are provided.