A Field Test and Analytical Study of Intermittent Gas Lift
Authors: Neely A.B. , Montgomery J.W. , Vogel J.V. 
Published: December 2007

A series of controlled experiments of intermittent gas life were carried out in an instrumented well in the Conroe field, Montgomery County, Tex. The well was equipped with seven pressure transducers over the length of the tubing string so that progress of the lifted slug of liquid could be followed up the tubing. Unique to the experimental setup was a surface-controlled, bottom-hole hydraulic valve that allowed for letting a liquid load into the tubing, closing the valve, and isolating the well from the controlled test. Thus, individual intermittent slugs could be studied independent of the well performance. A wide range of slug sizes and injection gas volumes was covered in the 52 test runs.Understanding the action of the gas-life valve is quite important in predicting intermittent-gas-lift performance. Gas-Lift valve action depends somewhat performance. Gas-Lift valve action depends somewhat on the pressure and forces acting upon the stem of the valve. Gas-life literature has assumed that this pressure is equal to the pressure in the casing once pressure is equal to the pressure in the casing once the valve opens. Tests carried out as a result of valve action seen in the instrumented well clearly indicate that this assumption is not valid. Some pressure between the casing pressure and the tubing pressure between the casing pressure and the tubing pressure will exist on the valve stem when the valve pressure will exist on the valve stem when the valve is open; and it is extremely important to be aware of this in predicting valve action. Some design techniques predict the amount of solid liquid slug that will be predict the amount of solid liquid slug that will be brought to the surface and assume that any additional liquid produced in the afterflow of gas will be negligible. It was observed in these tests that a significant portion of the liquid produced at the surface sometimes as much as 50 percent was contributed by this afterflow there will be a considerable discrepancy between predicted and actual results.Liquid recovery from individual runs did not correlate directly with any of the measured parameters. However, it appears that the amount of the liquid slug that is not produced can be correlated with the average gas velocity produced can be correlated with the average gas velocity below the slug. Since the starting slug size is known, the correlation can be used as a predictive technique in intermittent-gas-lift design. The design method has been compared with a field test.

Introduction

Although intermittent gas lift has been used for artificial lift in oil wells for many years, little concrete technology has been developed for it. Design methods and behavior predictions are as much an art as a science. There have predictions are as much an art as a science. There have been two major attempts to remedy the situation. White et al. attempted to analyze the motion of a finite slug of liquid propelled to the surface by gas injected at high pressure underneath. Supporting their premise by a modicum pressure underneath. Supporting their premise by a modicum of experimentation, they published results in the form of design curves. Brown and Jessen, on the other hand, attempted no analytical solution, but did extensive field testing to develop an empirical foundation for intermittent-gas-lift technology. Unfortunately, there was considerable discrepancy in the results of the two studies.To improve the technology in intermittent gas lift, Shell Oil Co, ran a series of controlled experiments in a gas-life well in the Conroe Field, Montgomery County, Tex. The well instrumentation necessary to carry out the tests is shown in Figs. 1 and 2. (The instrumentation technology was provided by B. C. Sheffield of Shell Development Co.)To predict intermittent-lift behavior, analytical methods are needed to calculate the time rate behavior of the casing gas pressure and volume, the flow of gas through a gas-lift valve, the velocity with which a liquid slug will be raised to the surface by this gas, the amount of liquid that will be produced at the surface and the amount left behind, the pressure gradients during the process, and the time decay pressure gradients during the process, and the time decay curve for the blowdown of gas pressure after the slug has surfaced. None of these functions is independent of the others and all must be considered simultaneously in predicting lift behavior.

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