Determining the Most Profitable Gas Injection Pressure for a Gas Lift Installation (includes associated papers 13539 and 13546 )
Authors: Blann J.R. , Williams J.D. 
Published: December 2007

Summary
Gas injection pressure has a very decided effect on the efficiency and operation of a continuous flow gas lift well. Selection of a gas injection pressure that is too high can result in needless investment in compression and other equipment, whereas pressures that are too low can cause in efficient gas lift operations and failure to produce a well's full potential. This paper discusses the produce a well's full potential. This paper discusses the effect of various producing parameters on the selection of gas injection pressure and describes techniques for predicting and evaluating these effects on a specific gas predicting and evaluating these effects on a specific gas lift installation to determine the most profitable operating system.

Introduction
The function of injection gas in a continuous flow gas liftwell is two-fold. First, it must aerate the fluid sufficiently to unload the well column down to an operating point. Second, it must reduce the fluid column density sufficiently to allow the reservoir pressure to push the produced fluid to the surface. The depth at which the produced fluid to the surface. The depth at which the operating valve is located depends on several factors, but primarily it is a function of the available injection gas primarily it is a function of the available injection gas pressure. pressure. To understand the current situation in the U.S. regarding injection gas pressures, it is necessary to examine the history of gas lift. In the early days (1865 to 1925), very large air stations were built and air was compressed and used to provide gas for artificial lift. The early compressors were driven by steam, which was generated in boilers burning crude oil. Later, compressors were driven by oil combustion engines and, in some areas, electric motors. These early compressors were usually single or dual stage and since they were made for a very low suction pressure (atmospheric pressure), there sulting discharge pressure was relatively low, usually on the order of a few hundred psi. In these early days, very little gas lift equipment mas available for use inside the well. Thus, the depth of lift was strictly a function of the depth to which the produced fluid column could be balanced, or slightly overbalanced, by a column of air. This meant that with a 600-psi[4.1-MPa] injection gas pressure at the surface, a well could be unloaded and gas injected down to about 1,500to 2,000 ft [457 to 610 m] of depth. However, such a situation may not have represented too much of a problem at that time, since most of the producing wells were problem at that time, since most of the producing wells were rather shallow compared with today's standards.
During the 1920's, the oil industry began collecting and selling the gas associated with oil production. The operating pressure for most of the gas transmission systems was usually around 800 psi [5.5 MPa] or less. Therefore, as a natural adjunct to the gas sales system, some of the compressed natural gas was used for gas lift. This yielded gas with much better properties at a higher pressure than was available from the old air lift systems. pressure than was available from the old air lift systems. In most respects, even today, gas lift injection pressures are still governed by the gas sales system pressures are still governed by the gas sales system pressures. Downhole equipment has been developed pressures. Downhole equipment has been developed during the past 50 years that allows wells to be lifted deeper with the available pressure, but basically the surface injection pressures have remained dependent on the gassales system pressures. Even where large gas fields have been discovered with higher pressures, the source for gaslift has almost always been located downstream of the processing facilities leading to the gas sales system. processing facilities leading to the gas sales system. The in efficiency of such low-pressure gas lift systems in deeper wells was masked during the early years of gas lift by the low value of gas and the low cost of gas compression. In addition, a low demand for oil encouraged very low well rates that could be accomplished with very little pressure drawdown in the producing well. About 12 to 15 years ago, this situation changed and suddenly many gas lift systems were found to be grossly inadequate for producing the higher rates that were required.

How Gas Injection Pressure Affects Gas Lift Efficiency
In a continuous-flow gas lift system, injection gas is used to supplement formation gas (Fig. 1) and the gas from these two sources combines to reduce the overall density of the produced fluid column. A low-pressure gas, which must be injected high in the fluid column, can affect the density of the fluid only above the point that it is injected(Fig. 2). Therefore, high volumes of gas, injected high above the formation, are required to affect the pressure drawdown at the reservoir face. Like wise, a relatively small volume of gas injected near the depth of the reservoir can have a decided effect on the density of the fluid column above it and thereby result in a significant pressure drawdown at the reservoir, pressure drawdown at the reservoir, JPT
p. 1305