Control structure design for stabilizing unstable gas-lift oil wells

Active control of the production choke valve is the recommended solution to prevent casing-heading instability in gas-lifted oil wells. Focus of this work is to find a simple yet robust control structure for stabilization of the system. In order to find suitable control variables, a controllability analysis of the system with different candidate control variables and two alternative manipulated variables was performed. Moreover, to include robustness and performance requirements at the same time, the controllability analysis was extended to a mixed sensitivity H∞ optimization problem. A control structure using only the available topside pressure measurements was found to be effective to stabilize this system.