Linear and non-linear analysis of flow instability in gas-lift wells

Linear and non-linear analyses of flow instability in continuous gas-lift wells were performed in this study. The linear analysis is based on a modified gas-lift stability criterion that takes into account compressibility of the mixture below the injection point and is applicable to saturated reservoirs. The analysis of non-linear dynamics and stability of the well was performed using direct numerical integration in the time domain of the governing equations describing the gas-lift system. The transient gas-lift well model developed comprises of a model of transient three-phase gas–oil–water flow in the wellbore, a transient model of gas flow in the casing annulus, and a pseudo-steady flow model in the reservoir. The multiphase flow model used is based on the drift-flux theory. Stability boundaries predicted by both linear and non-linear analysis were compared with field data published in a previous study; both types of analysis reproduced the data. The effects of the main well design and flow parameters on the frequency and amplitude of the oscillations during heading in a typical gas-lift well were studied. It was found that flow instability results in the oil production loss, which depends on severity of heading. The largest reduction in oil production takes place in case of the most severe heading in the well (flow instability with the largest amplitude of production rate oscillations). An increase in the lift gas consumption is required to compensate for the production losses caused by heading. An increase in the depth of the injection point may result in heading and an increase in the operating costs caused by the increase in the lift gas consumption. An increase in the separator pressure has a destabilizing effect. At high separator pressures the well can experience two modes of instabilities: casing heading and density-wave oscillations.