Automatic Start-up and Control of Artificially Lifted Wells

Abstract

Artificial lift in oil wells using a downhole pump has received much attentionthe last decades and is established as a reliable and efficient liftingmethod. In particular, artificial lift using an electric submersible pump(EPS) has proven to be an important alternative in deep wells with highproduction rate requirements.This thesis investigate the problem of controlling the ESP intake pressureunder rigid ESP operational constraints and input requirements, using theESP frequency and choke position as control inputs.A mathematical model of a system containing ESP lifted wells is createdand a control strategy that fulfil the control requirements is proposed.The model is an extension of the ESP model developed by Amundsen(2010) andcontain four ESP lifted wells. Mathematical proof of open-loop stabilityis derived based on analysis of a linearized well model. Also, closed-loopstability for the linearized model is proved based on frequency analysis.The proposed control strategy is tested on the model in a range of differentcase study. The control system display a capability of minimizing thecontrol error while keeping the ESP operated within the given constraints,with an optimal combination of control inputs. Simulation results indicatethat the proposed control strategy is capable of handling a step-changein ESP intake pressure, ESP tripping, and ESP and booster-pump start-up/shut-down.