Abstract

Heading (flow instability) is a common problem in the operation of continuous flow gas-lift wells. One of the key elements, which affect the gas-lift stability, is the operating valve. Several operating gas-lift valve designs are currently used in practice. A new valve design, based on the use of a supersonic nozzle ("the nozzle-Venturi gas-lift valve"), was recently proposed in the literature. In most cases, it is very difficult to estimate apriori (at the design stage) how the type of the gas-lift valve and its design parameters affect the stability of a gas-lift system to be operated at a given operating condition. In this paper, the effect of operating valve design on gas-lift stability was studied. It is shown that the gas-lift stability maps proposed in an earlier study may facilitate the selection of the operating valve design. Different valve designs were considered: the conventional orifice and gas-lift valves, as well as the nozzle-Venturi valve. Gas-lift stability maps were constructed for each operating valve design. Advantages and disadvantages of the considered valve types are discussed from the point of view of the gas-lift stability. Based on the comparison of the gas-lift stability maps, recommendations on the selection of the operation valve type were elaborated.

Introduction

Flow instability (heading) is an important issue in the design and operation of continuous flow gas-lift installations[1,2]. Heading reduces oil production and may cause a number of operational problems (compressor or low pressure separator shutdown, the gas and liquid flow rate surges, excessive vibrations, difficulties in measurement of production rates during well testing). Reviews of studies on gas-lift instability can be found in the previous papers[3,4].

The key element of any gas lift system is the gas lift valve. Square-edged orifice valves are traditionally employed to provide gas injection into the tubing. For a specified productivity index and injection flow rate, gas-lift stability can be reached by reducing the orifice size. However, operating valves with a small orifice size may require a high pressure drop across the valve. Recently, Tokar et al.[5] proposed a new injection valve design, which is based on the use of a supersonic nozzle (the nozzle-Venturi valve). They demonstrated that critical flow conditions can be reached in this valve when the downstream pressure is only 10 % less than the upstream pressure. Under critical flow conditions, the injection flow rate becomes constant and is controlled by casing pressure only. Alhanati et al.[1] recommend using a conventional gas-lift valve (with a dome, piston and a stem tip), which operates in the trottling range, as one of the methods of gas-lift well stabilization. Existing gas-lift design methods[6] do not provide guidelines for selecting the operating valve design to insure stable operating condition.

The purpose of this study is to investigate the effect of operating valve characteristics on gas-lift stability. Advantages and disadvantages of the three above mentioned types of valve design are discussed from the point of view of gas-lift stability. A stability map[3,4] was constructed for each valve design. Based on the comparison of the gas-lift stability maps, recommendations on the selection of the operation valve type are given.

Gas-Lift Performance

A gas-lift well (Tables 1,2 and 3) designed to produce 5,000 bbl/d with the injection flow rate of 4 MMscf/d was used in this study to perform the stability analysis. The predicted gas-lift performance curves for wellhead pressures from 5 Kg/cm[2] to 40 Kg/cm[2] are shown in Fig.1. The Hagedorn and Brown correlation[7] was used in the well performance calculations.

For each wellhead pressure, the optimal gas injection rate was also calculated based on a GLR sensitivity study. The curve representing optimum injection flow rates for different wellhead pressures is also shown in Fig.1.