Transient Dynamic Characteristics of the Gas-Lift Unloading Process
Authors: Tang Yula , Schmidt Zelimir , Blais Roger 
Published: December 2007

Abstract
This paper presents a transient dynamic model and simulator to describe the complicated characteristics of the gas-lift unloading process. The model uses mass and momentum balance equations, a cocurrent and countercurrent multiphase flow mechanistic model, and the latest valve dynamic performance model to couple together as a system the reservoir inflow to the flows in the tubing, in the casing, through gas-lift valves and through the surface injection choke. The effect of liquid flowing back into the formation is studied for the first time.
Satisfactory numerical stability is achieved by adopting an appropriate difference scheme and iterative method.
Several example calculations are given to illustrate the characteristics of unloading, to rectify some incorrect concepts and to underscore the unreliability of conventional design methods.
Instability of gas-lift production is analyzed with this simulator. This simulator is more accurate than existing analytical criteria because it requires fewer assumptions. The simulator was developed for practical application in the field. Among the many functions this new model and simulator can perform gas-lift unloading design, gas-lift optimal design (when coupled with a gas injection allocation subroutine), stability analysis, and gas-lift operation troubleshooting.

Introduction
Gas lift is one of the most widely used artificial lift methods in the world. Continuous injection of high pressure gas from the surface into the production string lightens the liquid load upon the bottom hole, enabling a higher production rate. Normally, gas-lift designs assume steady state flow and much guesswork. Often, the empirical designs used to space and design the unloading and operation gas-lift valves result in multipoint gas injection, unstable production and low lift efficiency. In fact, all gas-lift wells are put into production through a transient process of unloading. There are several possibilities with conventional unloading design.
One possibility is that the well achieves stable production. But this stable condition may not necessarily be the one for which it was designed. It is not even certain that single point injection is occurring. The lift efficiency may be less than desired, and the optimal production point may not have been attained.
Another possibility is that the gas-lift production is unstable, with all the production parameters fluctuating in time. Obviously, this is not the condition the production engineers desired when designing the well. The following steps have been taken to rectify this problem. First, restricting the liquid production rate by using a small choke on the wellhead or the flowline Second, increasing the gas injection flow rate by enlarging the size of the gas injection choke at the wellhead. Third, retrieving the gas-lift valves and changing the valve port sizes and dome pressure settings. Fourth, pulling out the tubing string to adjust the valve spacing. Unfortunately, all of these steps cost time and money with no guarantee that they will rectify the problem. One of the difficulties is the inadequacy of using steady-state equations to approximate the inherently transient unloading process: the transient two-phase flow in the tubing, the transient two-phase reservoir response, the transient flow through the gas-lift valve, and the transient flow through the surface gas injection choke. Therefore, it is extremely important to study the process of unloading. Only such study provides a solid basis for optimal design, optimal gas allocation, or gas-lift troubleshooting.

Background
While the literature is full of articles that address the unloading process, few among them describe the transient characteristics. Shinta did the first work on transient simulation of the unloading process in 1989. Unfortunately, his is a pseudotransient model because it uses steady-state equations for each finite time interval. The calculated values at one time step are used as the starting parameters for the next step.
Capucci and Serra developed a true transient unloading model in 1991. P. 647^