On instability in gas lift wells and schemes for stabilization by automatic control

Abstract

In this paper, we present a simple nonlinear dynamic model that is shown to capture the essential dynamics of the casing-heading instability in gas lift wells despite the complex nature of two-phase flow. Using the model, stability maps are generated showing regions of stable and unstable settings for the production valve governing the flow of produced oil and gas from the tubing. Optimal steady-state production is shown to lie well within the unstable region, corresponding to an oil-production rate that cannot be sustained without automatic control. Three simple control structures are suggested that successfully stabilize the casing-heading instability in simulations, and more importantly in laboratory experiments. Copyright © 2008 Society of Petroleum Engineers.