Design and Application of the Gas Jet Ejector On Marginal Gas Wells
Authors: Clanton G.W. 
Published: December 2007

This paper discusses the theory, design and application of gas jet ejectors for use on marginal gas wells. A high pressure gas well is used as a source of motive energy. Pressure energy is converted into kinetic energy through the special configurations of the nozzle and diffusor sections of the ejector. A method is given for the design of a gas jet ejector for natural gas applications. Thermal efficiency is inherently low due to the nature of the process; therefore, some kinetic energy is converted into heat which aids in the prevention of hydrates. The ejector offers a low-cost method of compression of natural gas in a field where motive energy is available. Proper design should prevent the formation of hydrates, without utilizing an additional source of heat, for moderate to high temperature flowing wells.

Introduction

An ejector is a device which converts pressure energy of a motive stream into kinetic energy, entrains a secondary stream and discharges the combined stream at an intermediate pressure. Ejectors have been in use for some time on other applications, particularly for steam and air service. Until the last few years, however, their use was limited for natural gas service-partially because of the limited amount of available thermodynamic data for natural gas. Gas wells with flowing pressures that approach the pipeline pressure have a tendency to load up with either the liquid condensate or water that they produce. By using an ejector, the wellhead flowing pressure can be effectively lowered, thereby increasing the velocity in the tubing and keeping the well unloaded. This critical velocity for entrainment of liquid in vertical tubing has been estimated to be between 5 and 10 ft/sec. A successful application of the ejector was made in offshore Louisiana with two gas wells on connecting platforms. One of the wells was marginal and would not flow continuously against separator pressure. With the aid of the ejector it was maintained on production without having to unload the well periodically to the atmosphere. In addition to providing a low pressure commingling header, the ejector discharged the combined stream at a lower temperature than would have been possible with an ordinary choke. Thus, the heat load on the glycol dehydration equipment was reduced.

Principle of Operation

The gas jet ejector works on the principle of entrainment. A high pressure motive gas stream is expanded through a converging-diverging nozzle into a suction chamber. The motive gas picks up a low pressure secondary stream by entrainment and compresses it to an intermediate discharge pressure. Two basic facts peculiar to gas behavior determine the operating principle of an ejector.

In a converging section, gases entering at less than the speed of sound are accelerated in velocity and reduced in pressure. At entering velocities higher than the speed of sound, gases are reduced in velocity and increased in pressure. In a diverging section, gases entering at less than the speed of sound are reduced in velocity and increased in pressure.

At entering velocities higher than the speed of sound, gases are increased in velocity and reduced in pressure. In Fig. 1, the velocity-pressure changes are depicted for a typical gas ejector.

JPT

P. 419ˆ