Increasing Production by Applying Active Slug-Suppression Technology
Authors: Carpenter Chris 
Published: October 2016

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 170731, “Increasing Production by Applying Field-Proven Active Slug- Suppression Technology,” by C.E. Lacy, SPE, G.A. Groote, and R. Chao, Shell; E. Osemwinyen, Nigeria NLNG; and F.A. Fleyfel, SPE, and O. Akinmoladum, Shell, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed.

Active slug-control methods have been developed in recent years. One such system is a relatively inexpensive solution developed by Shell that has proved to be very effective in stabilizing slugging in flowlines and risers. Field data from case histories in the Gulf of Mexico (GOM), Malaysia, and Nigeria indicate that production gains of 10% are often possible.

Introduction

The active slug-control method in question uses a single standard control valve and pressure sensors immediately upstream and downstream of the valve. One advantage of this method is that a choke/ control valve is often already installed. If changes are required, these are often limited to the trim and actuator. Because only localized pressure measurements are required, this makes the technology a cost-effective solution with very limited added weight or space requirements. Another advantage is that the system acts on both liquid and gas surges of the multiphase fluid without the need for prior separation. It can thus stabilize the fluids arriving at the production facility, allowing for optimization and increased production.

In this paper, a new application in Nigeria is discussed in which this active slug-suppression technology reduced the gas and liquid surges and lowered riser gas lift requirements. Riser-base gas lift is often used both to reduce hydrostatic head and to mitigate slugging in subsea applications. In this case, the asset was gas lift constrained and the gas lift was insufficient to mitigate slugging below a fixed flow-rate range. Lowering gas lift requirements meant more barrels could be produced across the asset. Reducing slugging would enable lower minimal production limits.

Technology

The concept of active slug control is several decades old. This latest technology, however, uses a single control valve— typically the boarding choke—located between the outlet of the riser and the first stage separator. A control algorithm especially designed for slug control was developed that incorporates the measurements of the pressure upstream and downstream of the control valve.

The first objective of the control system is to suppress riser-induced slugging. Here, riser-induced slugging (sometimes referred to as severe slugging) is characterized by the formation of a blockage beginning at the riser base. As the blockage forms, the gas production starts to decrease at the top of the riser and may stop completely. Eventually, a liquid slug arrives at the facility followed immediately by a gas surge. Once the gas surge is produced, the cycle then repeats itself. The control system can be used to minimize the impact coming from many types of transient slugs. Transient slugs can occur for a variety of reasons. These include start-up or ramp-up slugs and slugs resulting from dips in the elevation profile allowing alternating accumulation and blowout of liquids and slugs from instabilities either in the horizontal flowline or vertical riser.