Chapter 9 - Design of continuous gas lift installations

Abstract

The unloading process of a gas lift well is explained at the beginning of the chapter with the use of pressure–depth diagrams for each stage of process. Specific recommendations to unload the well without damaging the gas lift valves are given for different types of wells. Three procedures for calculating the operating injection point depth are explained in detail: (1) iterative procedure in which, based on nodal analyses, the operating point of injection depth, the target injection gas flow rate, and the liquid flow rate the well can produced, are determined; (2) fixed drawdown or fixed liquid production (taking the IPR curve into consideration): the procedure calculates, for a given injection gas flow rate introduced by the user, the injection point depth that makes the outflow curve intersect the IPR curve at the desired liquid flow rate (or the liquid flow rate for a given desired drawdown); and (3) fixed liquid production (without taking the IPR curve into consideration): the program calculates the unloading mandrel depths and the final point of injection depth, for a specific set of liquid production and gas injection rates, by simply applying any of the mandrels spacing procedure given in this chapter (the point of injection depth is the one found at the end of the mandrel spacing procedure). The following mandrel spacing procedures are explained in detail: (1) spacing by sequentially dropping the valve’s closing pressure, (2) spacing by sequentially dropping the valve’s operating pressure, (3) the “max–min” method, (4) the method for IPO valves with upstream chokes, and (5) the method for PPO valves. For each spacing procedure and unloading valve, the following variables are calculated: unloading liquid flow rate, required injection gas flow rate, seat diameter, unloading valve temperature calculation, and test-rack calibration pressure. Additional topics include: designs procedures for dual wells, redesign procedures, mandrel spacing from the reservoir static liquid level, stability check of the gas lift design, and several numerical examples of gas lift designs.