Natural Gas Fundamentals

7/21/2019 Natural Gas Fundamentals

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Gas Technology Lectures4th Year/Petroleum Refinery Engineering Branch

ByProf. Neran K. IrahimN!T"R!L G!#

$"N%!&ENT!L#'rigin ( )orl* Reser+es

Natural gas is generally considered a nonrene,ale fossil fuel. Natural

gas is called a fossil fuel because most scientists believe that natural gas

was formed from the remains of tiny sea animals and plants that died

200-400 million years ago.

Natural gas exists in nature under pressure in rock reservoirs in theEarths crust! either in con"unction with and dissolved in heavier

hydrocarbons and water or by itself. #t is produced from the reservoir

similarly to or in con"unction with crude oil. Natural gas has been formed

by the degradation of organic matter accumulated in the past millions of

years. $wo main mechanisms -iogenic an* thermogenic are

responsible for this degradation

%aw natural gas comes primarily from any one of three types of gas

wells.

&' crude oil wells

2' gas wells(' condensate wells.

Natural gas wells average 000 feet *ee1.

Natural gas produced from geological formations comes in a wide array

of compositions. $he varieties of gas compositions can be broadly

categori)ed into three distinct groups*

-2 Nonassociate* gas that occurs in conventional gas fields!

-3 !ssociate* gas that occurs in conventional oil fields!

- 5ontinuous -or uncon+entional gas. +ome types of unconventional

gas resources include6tight gas7

6coale* methane

6natural gas from geo1ressuri8e* a9uifers:7

6gas hy*rates:7

6*ee1 gas:7

,f the unconventional gas sources! the one most important to the gas


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transportation industry is coal e* methane. igure &' shows the world

natural gas reserves.

N/$%/1 /+ %E+E%3E+

Natural gas is used primarily as afuel an* as a ra, material inmanufacturing! ig. 2'.200

$ig.

Natural gas can be used in5

6ower generation

7ydrogen production

3ehicles

%esidential domestic use

ertili)er

/viation

Natural gas is also used in the manufacture of fabrics! glass!

steel! plastics! paint! and other products.

5om1osition

Natural gas is a complex mixture of hy*rocaron and non;hy*rocaronconstituents and exists as a gas under atmospheric conditions.

%aw natural gas typically consists primarily of methane -5


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pentanes and even higher molecular weight hydrocarbons.

8hen processed and purified into finished by-products! all

of these are collectively referred toNGL -Natural Gas

Li9ui*s.

!ci* gases* carbon dioxide -5'3! hydrogen sulfide -


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Gas #1ecifications;arket sales of natural gas re9uire some specifications set by the

consumers regarding the maximum contents allowable for the following*

acidic gases and sulfur! oxygen and carbon dioxide! water vapor! and

li9uefiable hydrocarbons. $he thermal heating content of the gas setsanother value to be met as a minimum.

#rrespective of the source of natural gas! the final specifications set formarket sales

re9uirements are usually the following*

Effect of Im1urities $oun* in Natural Gasield processing operations of natural gas! which is classified as a part of


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gas engineering! generally include the following*

2. Remo+al of ,ater +a1or: *ehy*ration

3. Remo+al of aci*ic gases -


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$ig.-? Pressure;tem1erature *iagram for a ty1ical natural gasmi@ture.

Natural Gas Pro1erties 5hemical an* Physical Pro1erties

Natural gas is colorless! odorless! tasteless! shapeless! and lighter than air

$able &'. $he natural gas after appropriate treatment for acid gas

reduction! odori)ation! and hydrocarbon and moisture dew point

ad"ustment would then be sold within prescribed limits of pressure!

calorific value! and possibly )oe in*e@ often referred to as the )oenumer.

Tale-2?


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Gas;#1ecific Gra+ity

+pecific gravity of gas is defined as

8hereMair is the molecular weight of air! which is e9ual to 2


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8hereMi is the molecular weight of component i!yi is the mole fraction

of component i! and n is the total number of components.

3arious gas properties! including the molecular weights for pure

components! are given in $able 2.

3. I*eal an* Real Gas La,s

$he volume of a real gas is usually less than what the volume of an ideal

gas would be! and hence a real gas is said to be supercompressible. $he

ratio of the real volume to the ideal volume! which is a measure of the

amount the gas deviates from perfect behavior! is called the

supercompressibility factor! sometimes shortened to the compressibilityfactor. #t is also called the gas deviation factor and is given the symbol =.

$he real gas e9uation of state is then written as*

whereP is the pressure! V is the volume! T is the absolute temperature!Z

is the compressibility! n is the number of kilomoles of the gas! andR is

the gas constant.$he theory of corresponding states dictates that theZ factor can be

uni9uely defined as a function of reduced pressure and reduced

temperature. $he reduced pressure and reduced temperature are defined

as

-----------------------


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wherePr and Tr are reduced pressure and reduced temperature!

respectively! andPc and Tc are critical pressure and critical temperature

of the gas! respectively. $he values of critical pressure and critical

temperature can be estimated from the following e9uations if the

composition of the gas and the critical properties of the individual

components are known*

wherePci and Tci are the critical pressure and critical temperature of

component i! respectively5 andyi is the mole fraction of component i.

$he values of critical pressure and critical temperature can be estimated

from its specific gravity if the composition of the gas and the critical

properties of the individual components are not known. +utton &?'

used regression analysis on raw data to obtain the following second-order

fits for the pseudocritical properties*

$hese e9uations are valid over the range of specific gas gravities with

which +utton &?' worked 0.? < @g < &.A>.

$he most commonly used method to estimate theZ factor is the chart

provided by +tanding and Bat) &


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Fig. (4): Compressibility of natural gases as a function of reduced

pressure and temperature +tanding and Bat)! &


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4. Gas %ensity

>. Isothermal 5om1ressiility of Gases

$he isothermal gas compressibility! which is given the symbol cg! is a

useful concept is used extensively in determining the compressible

properties of the reservoir. $he isothermal compressibility is also called

the bulk modulus of elasticity. as usually is the most compressible

medium in the reservoir. 7owever! care should be taken so that it is notconfused with the gas deviation factor!Z! which is sometimes called the

supercompressibility factor*

where V andP are volume and pressure! respectively! and T is the

absolute temperature. or ideal gas! we can define the compressibility as

whereas! for nonideal gas! compressibility is defined as

. Gas Aiscosity

Dust as the compressibility of natural gas is much higher than that of oil!

water! or rock! the viscosity of natural gas is usually several orders of

magnitude lower than oil or water. $his makes gas much more mobile in

the reservoir than either oil or water. %eliable correlation charts are


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available to estimate gas viscosity! and the viscosity of gas mixtures at

one atmosphere and reservoir temperature can be determined from the gas

mixture composition*

where ga is the viscosity of the gas mixture at the desired temperature

and atmospheric pressure!yi is the mole fraction of the ith component! i

is the viscosity of the ith component of the gas mixture at the desiredtemperature and atmospheric pressure!Mgi is the molecular weight of the

ith component of the gas mixture! andN is the number of components in

the gas mixture.

B!#I5 5'N5EPT# '$ N!T"R!L G!#

PR'5E##INGINTR'%"5TI'N%aw natural gas after transmission through the field-gathering network

must be processed before it can be moved into long-distance pipelinesystems for use by consumers. $he ob"ective of gas processing is to

separate

natural gas! condensate! noncondensable! acid gases! and waterfrom a gas-producing well and condition these fluids for sale or disposal.

$he typical process operation modules are shown in igure &. Each

module consists of a single piece or a group of e9uipment performing aspecific function. /ll the modules shown will not necessarily be present

in every gas plant. #n some cases! little processing is needed5 however!

most natural gas re9uires processing e9uipment at the gas processing

plant

1) to remove

impurities!


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water! and excess hydrocarbon li9uid2) to control delivery pressure.

PR'5E## &'%"LE#

1. $he first unit module is the1hysical se1aration of the *istinct1hases: which are typically

gas! li9uid hydrocarbons! li9uid water! and:or solids.

6hase separation of the production stream is usually performed in an inlet

separator.

igure &. +implified typical onshore treatment process.

7ydrocarbon condensate recovered from natural gas may be shipped

without further processing but is typically stabili)ed to produce a safe

transportable li9uid. nstabili)ed condensates contain a large percentage


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of methane and ethane! which will vapori)e easily in storage tanks.

$he next step in natural gas processing is aci* gas treating.

#n addition to heavy hydrocarbons and water vapor! natural gas often

contains other contaminants that may have to be removed. 5aron

*io@i*e -5'3: hy*rogen sulfi*e -


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BP Statistical Review of World

Energy June 2013b!co"#statisticalreview

Natural gas

World natural gas consu"tion grew by 2!2$% below t&e &istorical average of 2!'$! (onsu"tion growt& was above

average in Sout& ) (entral *"erica% *frica% and +ort& *"erica% w&ere t&e ,S -./!1$ recorded t&e largest

incre"ent in t&e world! n *sia% (&ina -.!$ and Jaan -.10!3$ were resonsible for t&e net4largest growt&

incre"ents! 5&ese increases were artly o6set by declines in t&e E, -42!3$ and t&e 7or"er Soviet ,nion -7S, -4

2!8$! 9lobally% natural gas accounted for 23!$ of ri"ary energy consu"tion! :E(; consu"tion grew "oreraidly t&an non4:E(; consu"tion for t&e < rst ti"e since 2000! 9lobal natural gas roduction grew by 1!$! 5&e

,S -./!'$ once again recorded t&e largest volu"etric increase and re"ained t&e world=s largest roducer! +orway

-.12!8$% >atar -.'!?$% and Saudi *rabia -.11!1$ also saw signi< cant roduction increases% w&ile Russia -4

2!'$ &ad t&e world=s largest decline in volu"etric ter"s! 9lobal natural gas trade was very wea@% growing by Aust

0!1$ in 2012! Pieline s&i"ents grew by 0!$% wit& declines in net Russian eorts -412$ artly o6set by growt&

in +orwegian eorts -.12$! ,S net ieline i"orts droed by 1?!?$! 9lobal C+9 trade fell for t&e < rst ti"e on

record -40!$D a decline in net Euroean C+9 i"orts -42?!2$ was o6set by net increases in *sia -.22!?$!

*"ong eorters% an increase in >atari -./!'$ s&i"ents was nearly o6set by a decline in ndonesia -41/!'$!

C+9=s s&are of global gas trade declined slig&tly to 31!'$!


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Reserves-to-production (R/P) ratios

World roved natural gas reserves at end42012 stood at 1?'!3 trillion cubic

"etres% sucient to "eet !' years of global roduction! Proved reserves

declined by 0!3$ relative to end42011 data% t&e


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World natural gas roduction increased by 1!$ in 2012! 5&e ,S once again recorded t&e largest national increase! Production grew

in every region ecet Euroe ) Eurasia% w&ere declines in Russia and t&e ,F o6set a gain in +orway! +atural gas consu"tion


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increased by a below4average 2!2$! *s was t&e case wit& roduction% t&e ,S recorded t&e largest national increase and

consu"tion rose in every region ecet Euroe ) EurasiaG E, consu"tion fell to t&e lowest level since 2000!


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Natural Gas Fundamentals

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