8/12/2019 MANNING Digitalizado

1/1

Chapter 2

Characterization of Natural Gas and Its Products

As indicated in Chapter 1, the conditioning of natural gasfor transportation and sale involves two process objectives:

1. Separation of the natural gas from free liquids (crudeoil, brine) and entrained solids (sand),

2.Removal of impurities from the natural gas and anycondensate formed to meet sales/reinjectionspecifications while observing all environmental

regulations.Sales specifications can be described most readily in terms

of the composition and properties of the producedhydrocarbons. Also, the selection, design, and operationof the processes required to separate gas from liquid andto remove impurities depend on the wellstream properties.Accordingly, natural gas and its products are discussedusing the following topics: spectrum of producedhydrocarbon fluids, natural-gas constituents, natural-gas

compositions, heating values, analysis of natural gases,sampling, and product specifications.

SPECTRUM OF PRODUCED HYDROCARBONFLUIDS

The desirable constituents of crude oil and natural gas arehydrocarbons. These compounds range from methane (CH4)

at the low-molecular-weight end all the way up to paraffinhydrocarbons with 33 carbon atoms and poly nucleararomatic hydrocarbons with 20 or more carbon atoms (Hatchand Matar, 1977).Natural gas is principally methane. Crudeoil is principally liquid hydrocarbons having four or morecarbon atoms.

There is a tendency to regard crude oil as a liquid andnatural gas as a gas and to consider production of the two

phases as separate operations. However, in the reservoir,crude oil almost always contains dissolved methane andother light hydrocarbons that are released as gas when the

pressure on the oil is reduced. As the gas evolves, the

remaining crude-oil liquid volume decreases; thisphenomenon is known asshrinkage. The gas so producedis called associated or separator gas or casinghead gas.Shrinkage is expressed in terms of barrels of stock-tankoil per barrel of reservoir fluid. Crude-oil shrinkage is thereciprocal of oil formation volume factor (FVF).

Similarly, natural gas produced from a gas reservoir maycontain small amounts of heavier hydrocarbons that are

separated as a liquid called condensate. Natural gascontaining condensate is said to be wet. Conversely, if nocondensate forms when the gas is produced to the surface,the gas is called dry.

A spectrum of well fluids is actually produced, as notedby McCain (1973) and summarized in Table 2-1. The typeof fluid produced depends on the phase diagram of thereservoir fluid and the reservoir temperature and pressure,as will be discussed in Chapter 3, Phase Behavior of Natural

Gas.Figure 2-1 depicts a typical gas-oil separation sequence

(including incidental water and sand removal). Table 2-1lists the five common types of wellstream fluids andsummarizes typical yields and liquid properties. When crudeoil is separated from its associated gas during production,the total gas evolved while reducing the oil to atmospheric

pressure divided by the volume of the remaining crude oilis called thegas-oil ratio or GOR. The GOR is expressed

as the total standard cubic feet of gas evolved per 60Fbarrel of stock-tank or atmospheric-pressure oil (scf/bsto),in English engineering units and standard cubic meters ofgas per cubic meter (or metric ton) of 15C oil in SI ormetric units. Standard conditions for natural gas are 60Fand 1 atm (English engineering units) and 15C and 1 atm(SI or metric).

The total GOR depends on the number of stages usedin the separation sequence, as well as the operating pressureof each stage. For three or more stages, the GOR approachesa limiting value. Optimization of the separation sequenceusually involves either maximizing crude-oil yield or