Combining Plunger Lift and Gas Injection in Low-Rate Gas Wells

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 100590, "The Tools, Techniques, and Advantages Involved in Combining Compression and Plunger Lift in Low-Rate Gas Wells," by R.A. Schmitz, SPE, and G.D. Steele, SPE, VaporTech Energy Services Inc., prepared for the 2006 SPE Gas Technology Symposium, Calgary, 15–17 May. Plunger lift and gas injection are two common methods used in gas wells to unload liquids. As the productivity of gas wells in the western Canadian sedimentary basin continues to decline, these methods used alone are no longer sufficient for many wells. Combining plunger lift with gas injection by use of newly developed and innovative tools and techniques can provide a solution for many low-rate gas wells that otherwise could not be cost-effectively produced. Introduction The maturing of the western Canadian sedimentary basin and the decline in average gas-well production rates, combined with strong demand and high commodity prices for natural gas, has led to a need to find ways to produce gas wells at lower and lower rates before they are shut in and abandoned. As a result, tools and techniques to remove liquids from these gas wells have become increasingly important. Both plunger lift and gas injection are techniques that have been used for many years to remove liquids from gas wells. Although there are several synergistic effects that occur when plunger lift and compression are combined, there also are a number of operational issues that make it challenging to combine the two technologies. Liquid Loading Liquid loading in a gas well is caused by the inability of the produced gas to remove the produced liquids from the wellbore. The point at which liquid loading will begin to occur in a producing well can be estimated by calculating the critical gas velocity, which can be translated into a critical gas flow rate. Various methods are available to estimate the critical gas flow rate, but the most important variables that determine the critical gas flow rate are gas production rate, inside diameter of the tubulars through which the gas is flowing, and the pressure and temperature of the gas. If the produced gas is flowing at a rate less than the critical rate at any point in the wellbore, liquid loading will occur. The produced liquid will start to accumulate in the wellbore, which often leads to erratic slugging flow and to decreased production. Eventually, the liquid buildup will cause the well to cease production entirely. Plunger Lift Fig. 1 shows a basic plunger-lift system. The primary function of the plunger is to provide a semisealing interface between the gas and the liquid, enabling the gas to lift the plunger along with a slug of liquid. The gas flow rate required for a plunger-lift system to function effectively depends on many of the same factors as the critical rate, but it also is affected by other factors such as the amount of liquid produced, the depth of the well, and the sealing efficiency of the plunger. As a result, prediction of gas-well performance with plunger lift is not an exact science.