The Use of Subsea Gas-Lift in Deepwater Applications
Authors: Jayawardena Subash S. , Zabaras George J. , Dykhno Leonid A. 
Published: October 2008

Riser base gas-lift is one of several methods that have been proven to work in subsea developments for production enhancement. In addition, gas-lift can be used as an effective method to suppress severe slugging that usually occurs in flowlines with downhill inclination. In some cases, gas-lift can be used to increase feasibility of blowdown for hydrate prevention.

Meanwhile, the necessity of gas-lift application is not always obvious because gas-lift effectiveness depends on reservoir performance, fluid properties, seabed terrain, subsea architecture, and flowline and riser specifications. In many cases, gas-lift for production enhancement is only needed at late life production, when oil production rates are low and water rates might be high enough. It is clear that the necessity of gas-lift, optimal operability and system design should be assessed from various aspects, including flow assurance. Whether gas-lift suitable as an artificial lift method and where the lift-gas should be injected needs to be evaluated very early in the project development lift cycle. Often, decisions need to be made with a very limited analysis to screen different development concepts, and whether to use gas-lift or not is one such decision.

Based on the past experience of riser gas-lift applications for different deepwater subsea projects and based the associated multiphase flow phenomena, a generic set of guidelines was developed. These include when gas-lift is beneficial for production enhancement, for flow stability and for flow assurance. Also, factors affecting the selection of a location for lift-gas injection, and the nature of the lift-gas supply method (dedicated vs. shared gas-lift risers, insulated vs. noninsulated gas-lift systems and gas-lift system functionalities needed) are discussed.

Introduction

This paper discusses the riser base gas-lift used in deepwater, subsea oil production systems. The information presented here is drawn from currently producing subsea systems as well as from those that are still in design stage. This paper does not discuss downhole gas-lift used in DVA (direct vertical access) or subsea wells, where challenges with regard to gas-lift application may appear quite different from those that are in the riser base gas-lift. The primary focus of this paper is to describe how flow assurance concerns affect various engineering decisions, in designing a gas-lift system.

Riser base gas-lift is injection of a pre-determined rate of gas into the production flowline (or riser) on the sea floor. Obviously, this applied to oil producing systems only. This injected gas is provided from the host facility, through a gaslift riser. The reasons for gas-lifting can vary, but the most important with regard to flow assurance are:Production enhancementFlow stabilizationFlowline depressurization

Gas-lift for production enhancement is usually used for cases where water cut is high and/or the reservoir fluid GOR or reservoir pressure is low. Lift-gas decreases the liquid holdup in the riser, and thus reduces the hydrostatic pressure in the flowline.