A nonlinear, adaptive observer for gas-lift wells operating under slowly varying reservoir pressure

Abstract

A nonlinear observer that estimates well flow rates and downhole pressure based on topside measurements only has been presented in the literature. As demonstrated in laboratory experiments the observer is suitable for a coupling with conventional PI control for stabilization of slug flow. The design of the observer is based on a nonlinear model with a linear inflow relation, and demands knowledge of several uncertain well parameters, including the reservoir pressure and production index. In this paper we present an adaptive extension to the nonlinear observer that eliminates one uncertain parameter by estimating the reservoir pressure. The extended observer is shown to be globally uniformly asymptotically stable and retain the properties of the original observer.