Chapter 1 – Gas properties

A description of the hydrocarbon gases that are commonly used as lift gas in most gas lift installations is presented at the beginning of the chapter, including the compositional limits that are used to classify these gas mixtures as corrosive, sweet or sour, or capable of generating hydrate related problems. The general equation of state is presented for ideal and real gases with the use of the compressibility factor and the universal gas constant. The way in which the compressibility factor is calculated from the pseudocritical pressure and temperature of the lift gas mixture is presented. Dalton’s and Amagat’s laws are described and used to solve problems and find the partial pressure of a particular pure gas in the mixture. The molecular weight of a gas mixture is determined from the mole fraction of its individual components. The specific gravity of a mixture of natural gases is defined and different methods that can be used for its calculation are presented. The following additional topics are presented: general equations for the calculation of the gas viscosity, determination of lift gas solubility in water, procedure to determine the water vapor content of a mixture of hydrocarbon gases, procedures to predict the onset of hydrates, and calculation of the gas specific heat ratio.