Improved feedback control system of unstable gas-lift wells

Abstract

Oil production wells on gas lift are sometimes unstable at low gas lift rates, even though steady-state flow analysis gives most efficient production at these gas lift rates. Unstable production, often called heading, may lead to periods of reduced or even no liquid production followed by large peaks of liquid and gas. This result in average oil production less than expected, and oil and gas production less than the systems design capacity to allow for the production peaks without causing shutdowns. To solve the problem the amount of lift gas is normally increased beyond the most optimum rate. When the lift gas supply is limited, other gas lifted wells must then be shut in. This paper describes field proven sequence based automation for oil production, plus a new model-based automatic controller. Both technologies solve the problem of unstable production from gas lifted wells through manipulation of the production and/or the gas-injection chokes. The sequence-based automation and the model-based controller stabilize the production at operating points that would be unstable under standard operation. Field measurements such as wellhead pressure, annulus pressure etc. are inputs to the controllers (or are used by the controller). Examples are given of the modelbased controller working together with realistic transient flow models of gas lift wells. Testing of the model-based controller on real unstable wells is planned.