3.1 Introduction

Gas lifting is a process of lifting fluids from an oil well by the injection of gas (a) continuously in the upward-rising liquid column (in case of continuous flow gas lift), or (b) underneath an accumulated liquid slug in a relatively short period of time to move the slug to the surface (intermittent lift). In both cases, high-pressure gas (or air in early practice) from the surface is led downhole, either through the casing-tubing annulus, the tubing, or some special conduit. This gas is injected into formation fluids at predetermined depth or depths in the well. To achieve an efficient operation and to ensure that the proper amount of gas is injected at all times, gas entry must be controlled by utilizing some kind of a downhole control device. In modern practice, gas lift valves are used for downhole gas injection control.

The gas lift valve is the heart of a gas lift installation since it provides the necessary control of well production rates and its performance determines the technical and economic parameters of fluid lifting. Because of its importance, there have been a great number of varieties of gas lift valves developed over the years. The abundance of the different operating principles and technical features allows the operator to choose the proper type of valve for any case.

Chapter 3 covers all aspects of gas lift valves including their evolution, operating principles, performance, and application. Most of the discussion is related to the family of pressure-operated valves...