Cas lift efficiency is substantially affected by unstable behaviour of wells. Unstable production, or heading, results in prolonged periods of reduced or even vanishing production and therefore leads to significant oil deferment. Since heading can be caused by a large variety of factors, such as incorrect gas lift string design, improper valve setting, or valve leaking or plugging, it is often difficult to find the origin of the leading. To aid in the diagnosis of gas lift problems, a dynamic gas lfit simulator was developed. The simulator computes the transient flow of gas and liquid through the production tubing. Valve models are used to calculate the passage of gas or workver fluid through the valves as a function of pressures local to the valves. The simulator is used to study poor lift gas performance, quantify the effect of remedial measures and optimise unloading procedures. Examples are given of field cases that were studied with the simulator.
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