Dynamic Model of Gas-Lift Valve Performance
Authors: Hepguler Gokhan , Schmidt Zelimir , Blais R.N. , Doty D.R. 
Published: December 2007

A theoretical and experimental study was performed to develop a dynamic model for gas-passage performance of a 1.5-in., nitrogen-charged, bellows-operated gas-lift valve. Performance curves were obtained by using air for 0.25 and 0.50-in. ports with flow rates reaching 2.5 MMscf/D. Internal pressures and temperatures were measured during flow-performance tests to develop a dynamic model for both orifice and throttling flow.

Introduction

To design an efficient gas-lift installation, the production engineer needs reliable information on the performance of all system components, from the outer boundary of the reservoir to the separator. One critical component is the gas-lift valve. In a producing system, the gas-lift valve controls the point of entry of producing system, the gas-lift valve controls the point of entry of compressed gas into the production string and acts as a pressure regulator while the injection gas is controlled at the surface choke. During the unloading process, the behavior of gas-lift valves becomes the primary factor for reaching optimum single-point gas injection depth. Injection pressure-operated valves are the most commonly used continuous-flow gas-lift valves. They consist of a nitrogen-charged dome and bellows assembly connected to a stem and ball that seat on a port (Fig. 1).

The performance curves of injection pressure-sensitive valves show two distinct flow regions (Fig. 2). In the orifice flow region, at a constant injection pressure, the flow rate increases as downstream pressure decreases during subcritical flow, but eventually critical flow occurs, where flow rate remains constant despite further decreases in downstream pressure. On the other hand, in the throttling flow region, at a constant injection pressure, the flow rate increases with decreasing downstream pressure, the flow rate increases with decreasing downstream pressure until it reaches a maximum and then decreases with pressure until it reaches a maximum and then decreases with decreasing downstream pressure. For a given port size, the occurrence of orifice or throttling flow depends mainly on the relative magnitudes of the nitrogen pressure in the dome and the injection pressure.

One way to obtain reliable data in orifice and throttling flow regions is to perform flow-performance tests on the gas-lift valves currently available with the valve treated as a black box and volumetric flow rates reported as a function of valve-setting parameters and the differential pressure across the valve. This parameters and the differential pressure across the valve. This data-acquisition method is extremely time-consuming because of the combination of parameters affecting gas-passage performance of a valve. Modeling the valve on physics principles allows a significant reduction in the number of tests needed to characterize valve performance.

This study investigates pressure and temperature distribution within the valve, internal valve mechanism, and forces acting on internal elements of the valve, The paper explains the nature of the experimental data and results obtained, defines the important parameters that affect valve performance, and provides a model for parameters that affect valve performance, and provides a model for both orifice and throttling flow regions.