Behavior of gases_Gas Properties_OnG.pdf

All About Gases

1. BEHAVIOR OF GASES
Gas Properties

2. Definition of a gas
Agasisahomogeneousfluid,generallyoflowdensityandlowviscosity.
Gas has no definite volume but assumed the volume of any vessel which
it is placed.
Specificlawsthatexpressthebehaviorofgasesatvarioustemperature
andpressureareveryimportantinpetroleumtechnology.
Thegasesaredividedintoideal(perfect)gasandreal(non-ideal)gas.
Thepropertiesofhydrocarbongasesarerelativelysimplesincethe
parametersofpressure,Volumeandtemperature(PVT)canberelatedbya
singleequation.
Thebasicsforthisequationanadaptationofacombinationoftheclassical
lawsofBoyle,CharlesandAvogadro.
Intheequationofstateforanidealgas,thatisagasinwhichthevolumeof
thegasmoleculesisinsignificant,attractiveandrepulsiveforcesbetween
moleculesareignored,andmaintaintheirenergywhentheycollidewitheach
other.

3. Behaviour of hydrocarbon gases
•PV =nRT the ideal gas law
Where field units SI units
P = absolute pressure psia bara
V = volume cu.ft. m
3
n = number of moles of gas - -
T = absolute temperature
o
Rankine
o
Rankine
R = universal gas constant psia. Cu.ft. KJ/kmol.K
The above equation is valid at low pressure where the assumptions
hold.
However, at typical reservoir temperatures and pressures, the
assumptions are no longer valid, and the behaviour of hydrocarbon
reservoir gases deviate from the ideal gas law.

4. Behaviour of hydrocarbon gases
In practice, it’s convenient to represent the behaviour of these
“real” gases by introducing a correction factor known as the gas
deviation factor, into the ideal gas law:
PV = znRT the real gas law
The z-factrormust be determined empirically,(i.e. by experiment),
but this has been done for many hydrocarbon gases, and
correlation charts exist for the approximate determination of the
z-factor at various conditions of pressure and temperature.

5. The Perfect Gas Laws
Boyle’s Law: For a given weight of gas, at a given temperature, the
volume varies inversely as the pressure.1
V PV constan t
P
  
Charles’ Law (Gay-Lussac’s Law): For a given weight of gas, at a given
pressure, the volume varies directly as the absolute temperature.V
V T constan t
T
  
Absolute temperature:
o
R =
o
F + 460
o
K =
o
C + 273
Avogadro’s Law: Under the same conditions of temperature and pressure,
equal volumes of all perfect gases contain the same number of molecules
one lb-mole of any perfect gas occupies a volume of 379 standard cu ft (60
o
F, 14.7
psia), and one g-mole occupies a volume of 22.4 standard liters (0
o
C, 1 atm)

6. T
1= constant
(V
1P
1T
1) (V P
2T
1)
P
2= constant
(V
2P
2T
2)
Step 1
Step 2
Step 1: Boyle’s Law
Step 2: Charles’ Law11
1 1 2
2
PV
P V P V V
P
   2 2 1
1 2 2
V P TV
V
T T T
   1 1 2 1 1 1 2 2
2 2 1 2
P V V T P V P V
P T T T
  
Eliminating V in both equations:
Thus, for a given weight of gas,
P V
T
= constant
Combination of Boyle’s Law and Charles’ Law

7. Combination of Boyle’s Law and Charles’ Law
with Avogadro’s Law
Combination of all four laws
gives,PV
R
T
 wt
PV nRT or PV RT
MW

R = a gas constant, that has
the same value for all gases
If n moles of gas, and since n
is the weight of gas divided by
the molecular weight
alsoknown as
the general gas
law
P V T n R
atm liters
o
Kgrams/MW0.0821
atm cc
o
K grams/MW82.1
psia cu ft
o
RIb/MW 10.73

8. Density of a Perfect Gas
Gas Mixturesi
i
i
V
(Volume%) x100
V

 i
i
i
Wt
(Weight %) x100
Wt

 ii
ii
ii
nn
(Mole%) x100 molefraction, y
nn
  
 
g
n MW MW PWt
nRTV RT
P
   
Since density is defined as the weight per unit volume, the general gas
law can be used to calculate densities of gases at various temperatures
and pressures.
1.
2.
3.
4.(Volume %) = (Mole %)

9. Actualgasesapproachperfectgasbehaviorathightemperaturesandlow
pressures.Inaperfectgasthemoleculesthemselvesareassumedtobeof
negligiblevolume(comparedtothevolumeofthegas)andtoexertno
attractiveforcesononeanother.
Athighpressuresandlowtemperaturesthisisnotsosince,underthese
conditions,thevolumeofthemoleculesthemselvesisnolongernegligible
andthemolecules,beingmorecloselypacked,exertappreciableattractive
forcesononeanother
Van der Waals’ equation is used to describe non-perfect gas behaviors.  
2
a
P V b RT
V

  


Non-Perfect Gases
where: aand bare constants whose values are different for each gas,
a/v
2
accountsfortheattractiveforcesbetweenthemolecules,andisaddedtothe
pressurebecausetheactualpressurewouldneedtobelargertoproducethesame
volumethanifnoattractionexisted,andTheconstantbrepresentsthevolumeof
themoleculesthemselves,anditissubtractedfromVsincetheactualvolumeof
spaceavailabletothegasislessthantheoverallvolumeofthegas.WhenVislarge
(atlowpressureandhightemperatures),itisobviousthatVanderWaals’equation
reducestothegeneralgaslaw

10. where Z is known as the gas compressibility factor or gas z-factor.
Gasz-factorisanempiricalfactor,determinedexperimentally,which
makestheaboveequationtrueataparticulartemperatureandpressure.
Foraperfectgas,Zisequaltoone.Foranimperfectgas,Zisgreateror
lessthanone,dependingonthepressureandtemperature.
Thelawofcorrespondingstatesexpressedthatallpuregaseshavethe
samez-factoratthesamereducedtemperatureandpressure.
Gas Compressibility Factor
For an imperfect gas one can write the general gas law in the form PV znRT rr
cc
PT
P and T
PT
   
pc i ci pc i ci pr pr
pc pc
PT
P y P and T y T P and T
PT
    
where P
r= reduced pressure, T
r= reduced temperature, P
c= critical pressure,
and T
c= critical temperature
For an imperfect gas mixture

11. Gas Formation Volume Factor, B
go o o o
P V z nRT 1 1 1 1
PV z nRT
•B
gis defined as volume in bbl which is being occupied by 1 SCF gas
when the gas was brought to reservoir conditions or is defined as the
volume of gas at reservoir conditions which can produce 1 SCF of the
gas at surface standard conditions.
•B
gEstimation Method
V
o, T
o, P
o,
z
o, n
reservoir
V
1= 1 SCF
T
1= 60
o
F
P
1 = 14.7 psia
Z
1= 1.0
n
surface

12. Combining equations:o o o o
1 1 1 1
P V z T
P V z T
 o o o o o o o1
g
1 1 1 o o o
V z T z T (14.7) z TP
B 0.02826 cuft /SCF
V z T P (1)(460 60)P P
   
 o o o
g
1o
V 5.62 z T
But1bbl 5.62cuft, B 0.00504 bbl/SCF
VP
  

13. Gas Formation Volume Factor
[res bbl/SCF] or [ft
3
/SCF]SC
R
g
V
V
B
Volume of an arbitrary amount
of gas at reservoir T & P
Volume of SAMEamount at
standard T & P

14. Gas Formation Volume Factor
•Gas Formation Volume Factor is the volume in
barrels (cubic metres) that one standard cubic foot
(standard cubic metre) of gas will occupy as free
gas in the reservoir at the prevailing reservoir
pressure and temperature.

15. Gas Formation Volume Factor
Using equation of statePV znRT
andres res SC SC
res res SC SC
P V P V
z T z T
 SC R RR
g
SC R SC SC
P T zV
B
V P T z

Z at standard
conditions = 1.0

16. Gas Formation Volume Factor (McCain)

17. ReciprocalofBgoftenusedtoreduceriskof
misplacingdecimalpointasBgislessthan0.01g
1 volume at surface
E
B volume in formation

E is referred to as ‘Expansion Factor’
Gas Formation Volume Factor

18. Gas Formation Volume FactorSC
R
V
V
Bg G as Form ation V olum e Factor
Bg
Pressure P
znRT
V
R

19. Viscosity Of Gases
•Whenfluidflowinthereservoirisconsidered,itis
necessarytoestimatetheviscosityofthefluid,since
viscosityrepresentsaninternalresistanceforcetoflow
givenapressuredropacrossthefluid.
•Unlikeliquids,whenthetemperatureandpressureofa
gasisincreasedtheviscosityincreasesasthe
moleculesmoveclosertogetherandcollidemore
frequently.
•Viscosityismeasuredinpoise.Ifaforceofonedyne,
actingononecm
2
,maintainsavelocityof1cm/sovera
distanceof1cm,thenthefluidviscosityisonepoise.

20. Viscosity Of Gases
•But for practical purposes, the centipoise (cP) is
commonly used.
•The typical range of gas viscosity in the reservoir is
0.01 –0.05 cP.
•By comparison, a typical water viscosity is 0.5 –1.0
cP.
•Lower viscosities imply higher velocity for a given
pressure drop, meaning that gas in the reservoir
moves fast relative to oil and water, and is said to have
a high mobility.

21. Viscosity (cp)
Pressure
T increasing
100
o
F
150
o
F
200
o
F
100
o
F
150
o
F
200
o
F
Gas Viscosity

22. Gas Density
•Densityisthemostcommonlymeasuredpropertyofa
gasandobtainedexperimentallybymeasuringthe
specificgravityofthegas(densityofthegasrelativeto
air=1)
•Aspressureincreases,sodoesgasdensity,butthe
relationshipisnon-linearsincethedimensionlessgas
compressibility(z-factor)alsovarieswithpressure.
•Thegasdensity(ρ
g)canbecalculatedatanypressure
andtemperatureusingtherealgaslaw:
ρg=MP/zRT
WhereMisthemolecularweightofthegas(Ib/molor
kg/kmol)

23. Relationship between subsurface and surface
gas volumes
•Themostimportantuseoftherealgaslawistocalculate
thevolumewhichasubsurfacequantityofgaswill
occupyatsurfaceconditions,sincewhengassales
contractsarenegotiatedandgasissubsequentlysold,is
referredtoinvolumesatstandardconditionsof
temperature(T
sc)andpressure(P
sc).
•Therelationshiprequiredisthegasexpansionfactor(E),
andisdefinedforagivenquantity(massornumberof
moles)ofgasas;
E=volumeofgasstandardconditionsscf
vol.ofgasatreservoirconditionsrcf

24. Relationship between subsurface and surface gas volumes
•Itcanbeshownusingtherealgaslaw,andtheknowledgethatat
standardconditionsz=1.0,thatforareservoirpressure(P)and
temperature(T):
E = 1/z . T
sc/ T . P/ P
scvol. /vol.
Thepreviousequationisonlyvalidaslongasthereisno
compositionalchangeofthegasbetweenthesubsurfaceandthe
surface.ThevalueofEistypicallyintheorderof200,inother
wordsthegasexpandsbyafactorofaround200fromsubsurface
tosurfaceconditions.
Theactualvalueofcoursedependsuponboththegascomposition
andthereservoirtemperatureandpressure.Standardconditions
oftemperatureandpressurearecommonlydefinedas60oF(298K)
andoneatmosphere(14.7psiaor101.3KPa),butmayvaryfrom
locationtolocation,andbetweengassalescontracts.

25. Relationship between subsurface and
surface gas volumes
•Ingasreservoirengineering,thegasexpansion
factor,E,iscommonlyused.Butinoilreservoir
engineeringitisoftenconvenienttorefertothegas
formationvolumefactor,B
g,whichisthereciprocalE,
andisexpressedinunitsofrb/scf(usingfieldunits)
Hence
Bg (rb/scf) = 1/ 5.615 E

26. Gas Formation Volume Factor, B
g
B
gisplottedasafunctionofreservoirpressureandtemperatureat0.60,0.70,0.80,and
0.90gasgravities.Othergasgravities,B
gmaybeobtainedbyinterpolation.
B
g

27. Gas Solubility, R
s
•Thegassolubility(R
s)isdefinedasthenumberofcubicfeetofgasmeasured
atstandardconditionswhichareinsolutioninonebarrelofstocktankoilat
reservoirtemperatureandpressure,orsimplythevolumeofgasproducedat
thesurfacedividedbythevolumeofoilinthereservoirwherethegaswas
existed.UnitforR
sisSCF/STBorcuft/bbl.
•Gas solubility in crude oil increases with the increase of pressure until it
reaches the saturation pressure (P
b). At pressure above P
b, R
sis constant or
unchanged.
•Gas solubility decreases as the temperature increases.
•At constant pressure and temperature, gas solubility in crude oil will
decrease with the decrease of specific gravity of gas.
•At constant pressure and temperature, Gas solubility in crude oil will
increase with the increase of
o
API gravity of the crude oil.
•Gas solubility is dependent on the type of gas liberation processes. Flash
liberation process produces much bigger gas solubility as compared to
differential liberation one.
The gas solubility (R
s): main observations

28. Gas Solubility, R
s: Important observations
gas solubility for saturated crude oilgas solubility for under-saturated crude oil
Gas liberation processes effects on
gas solubility

29. Gas Solubility, R
s: Estimation Methodsoo
o60 F / 60 F
141.5
S.G.
API 131.5


•gas solubility correlation as function of pressure and
o
API