Subsea Gas Lift in Deepwater Applications

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18820, "The Use of Subsea Gas Lift in Deepwater Applications," by Subash S. Jayawardena, George J. Zabaras, SPE, and Leonid A. Dykhno, Shell Global Solutions, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May. The paper has not been peer reviewed. Riser-base gas lift is used in subsea developments for production enhancement. It is an effective method to suppress severe slugging that occurs in flowlines with downhill inclination. In some cases, gas lift can aid blow-down for hydrate prevention. Gas lift is not always needed because its effectiveness depends on reservoir performance, fluid properties, seabed terrain, subsea architecture, and flowline and riser specifications. The need for gas lift, optimal operability, and system design should be assessed from various aspects, including flow assurance. A generic set of guidelines was developed on the basis of past experience with riser gas lift applications for different deepwater subsea projects and associated multiphase-flow phenomena. Introduction This paper discusses riser-base gas lift for deepwater subsea oil-production systems. The focus is how flow-assurance concerns affect various engineering decisions in designing a gas lift system. Riser-base gas lift is injection of a predetermined rate of gas from the host facility into the production flow-line (riser) at the seafloor. The reasons for gas lifting can vary, but the most important pertain to flow assurance, production enhancement, flow stabilization, and flowline depressurization. Why Gas Lift Is Needed The stages of a field's life should be studied to determine when to install and operate the gas lift system. Gas lift is not always beneficial; in some cases, increasing the gas rate may be detrimental to the performance of the subsea system. Production Enhancement. Gas lift for production enhancement lowers the flowline pressure. Typically, gas lift is needed with high water cuts in the flowlines, low-GOR fluids, and low-to-moderate production rates. The effectiveness is higher in systems with low production-system-inlet pressures. One major advantage of gas lift for production enhancement is that there are no moving parts in the subsea system, apart from valves and chokes. Compression for gas export is almost inevitable in any subsea development; therefore, the supply of lift gas during production is not a major issue. When to use gas lift for production enhancement should be determined by use of integrated (reservoir/wells/flowline) production modeling. The study also should include the water/gas injection to the reservoirs. Accuracy of flowline/riser pressure-drop and liquid-holdup calculations in multiphase-flow models and the accuracy of pressure/volume/temperature predictions are crucial. This accuracy becomes more important in larger-diameter flowlines with deepwater risers in which the multiphase behavior is different from that in smaller-diameter systems.