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TUALP Abstract
 

Yula Tang (Master of Science in Petroleum Engineering)
Transient Dynamic Characteristics of Gas-Lift Unloading Process
(173 pp.)
Directed by Dr. Zelimir Schmidt, and Dr. Roger Blais
(358 words)

This study presents a new comprehensive transient dynamic model and simulator to describe the complicated characteristics of gas-lift unloading. The model makes use of conservation equations for mass, momentum and energy, of the co-current and countercurrent multiphase flow mechanism, of TUALP's latest valve dynamic performance model, of reservoir inflow and back:flow performance, and of single or two-phase flow through chokes. The simulator couples together as a system the different flows

in the tubing, in the casing, through gas-lift valves and a surface injection choke, as well as reservoir inflow.

For the first time transient temperature changes in the gas-lift unloading process, and their effect on unloading characteristics, are studied. The transient tubing pressure and two-phase flow parameters are incorporated into the temperature calculation. Two heat transfer mechanisms in the wellbore are considered. For natural convection where liquid in the annulus is at rest, an equal heat-rate model in the radial direction is used with only one energy equation. For the case where both the injection gas and annulus liquid are in motion, two energy equations describe the flow in the tubing and annulus respectively.

A double iteration procedure on both the temperature and pressure numerically solves the three conservation equations simultaneously. An explicit numerical scheme for the partial differential equations of mass and momentum is formulated with a staggered grid. The equations of energy are solved by analytical means. Rapid convergence and stability are achieved.

Practical situations are considered in the development of the simulator, including complex wellbore geometries (e.g., inclined wellbore, tapered tubing and casing strings) and a wide range of realistic operating conditions.

Examples are given to illuminate the characteristics of unloading, and to indicate some wrong concepts and the unreliability of conventional design methods. How to use the simulator to optimize the gas lift unloading design is illustrated. Gas lift production instability is analyzed with this simulator, which is more accurate than other analytical criteria because it is based on fewer assumption.

This study provides a new and powerful tool to investigate the design and operational characteristics for gas-lift wells. It helps to unload gas-lift wells more successfully and to operate them more cost effectively.

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Last updated September 04, 2012