8.1 Introduction

In order to reduce drilling and completion costs, often several productive formations are produced through a single borehole. This is especially desirable when many formations are located vertically close above one another, and considerable savings on drilling and completion costs can be realized if several zones are produced simultaneously. Although theoretically possible, multiple installations tapping into more than two zones are seldom produced by gas lift. However, dual gas lift, defined as the simultaneous production by gas lifting of two formations opened in the same well, is a time-proven part of gas lift technology. ^[1], ^[2], ^[3]

Generally, if several formations are produced from one well, the main rule is that each zone must be produced independently from the others. This is because only through continuous monitoring of the produced fluid amounts can reservoir engineers simulate the behavior of the field. Since commingling of wellstreams (as shown previously) is not recommended, dual gas lift installations have to contain two tubing strings dedicated to the two productive formations. The schematic of the generally used dual installation is shown in Figure 8-1, with two packers isolating the two formations. Both the short string producing the upper zone and the long string producing the lower zone are equipped with their individual operating valves and unloading valve strings. Both production strings share a common annulus, and this is the main source of many problems associated with dual gas lift.

Figure 8-1: Schematic of a dual gas lift installation.