PETE 310
Lecture # 14
Wet Gas – Specific Gravity & Z-factor
(Chapter 7: pages 195-205)
Learning Objectives
 Calculate the specific gravity of a wet gas
mixture, given producing GOR (at separator(s)
and stock tank and:
 compositions liquid and gas from stock tank
and separator gas
 or, separator compositions (gas & liquid)
 or, properties of the separator gas and stock
vent gas
 Define the two-phase z-factor and understand the
uses of this in reservoir engineering
 Explain the shape of a typical two-phase z-factor
isotherm.
 Calculate values of two-phase z-factor using
Rayes etal. correlation (SPE paper).
Separators
y iSP
and GOR ( scf / STB )
y iST
and GOR ( scf / STB )
x iST
xiSP
xiST
Wellhead
fv SP 
lb  mole 
lb  mole
gas
gas SP
 lb  moleoil SP
lb  moleoil SP  lb  molegas  lb  moleoil ST
fv ST 
lb  mole 
lb  mole
gas
gas ST
 lb  moleoil ST
Key Points
What matters is the molar ratio of gas to oil
so let’s assume one barrel of oil produced
Methods to evaluate oil density will be
discussed in Chapter 11 (here it will be
provided)
To convert oAPI to oil density
o
141.5
API 
 131.5
o
o
o 
w
Key Points
 The expression [=] means “has the units of…” For
example
lb
o  3
ft
 You are responsible for reading the material that
cannot be covered in this lecture
 Rework ALL the example problems in the book
 Procedure 1 - explained in detail here - is simpler
and takes less time to solve than the method
explained in the book
Recombination procedure when
separator gas yiSP and tock tank
compositions (xiSTO, yiST) are
known
(Procedure 1.)
Procedure 1.
Calculate molecular weight of stock tank
liquid
Nc
Mwo   x i Mwi
i 1
Calculate lb-moles of separator gas produced
per barrel of STO from separator

GOR SP
scf/STB 

 lb - molegas /STO
id
scf/lb - mole
Vm
Vmid  380.7scf/l b - mole (ideal gas molar volume)
Procedure 1.
Calculate lb-moles of stock gas vented per
STO

GOR ST
scf/STB 

 lb - molegas /STO
id
scf/lb - mole
Vm
Calculate moles of oil in 1 barrel of stock tank
(need to use molar density)


oil
lb - moleoil
lb/ft 3


lb/lb - mole
Mwo
ft 3
lb - moleoil
ft 3
 lb - moleoil /STO
 5.615
3
ft
bbl
Procedure 1.
lb  mole 
gas SP
fv SP
STO

lb  molegas  lb  moleoil SP
xiST
STO
lb  moleoil SP  lb  molegas  lb  moleoil ST
lb  mole 
gas ST
fv ST 
STO
lb  molegas  lb  moleoil ST
STO
Procedure 1.
 Determine reservoir gas composition from
fundamental mole balance

zi  y i SP fv SP  xi SP 1  fv SP


xi SP  y i ST fv ST  xi ST 1  fv ST

xiST

1  f 1  f 
zi  y i SP fv SP  y i ST fv ST  xi ST
v ST
v ST
 Once reservoir composition is known determine zfactor and specific gravity
Example for Procedure 1.
Yi SEP
Yi STO
X i STO
Recombination procedure when
separator gas yiSP and liquid
compositions xiSP are known
(Procedure 2.)
Example for Procedure 2.
Procedure 2.

zi  y i SP fv SP  x i SP 1  fv SP

 Additional information given is the separator/stock
tank volume ratio as
bbl SP oil at (T, P of separator)
bbl STB ( at standard conditions)
 Use this to convert from scf/STO  scf/ST
 Proceed as in procedure 1.
 Rework example 7.2 in textbook
Recombination procedure when
only separator gas and stock vent
gas properties are known
(Procedure 3.)
Procedure 3.
For two-stage separators
g 
RSP  gSP  RST  gST
RSP  RST
R  RSP  RST
For three-stage separators … derive
expressions
Procedure 3.
Moles in one stock tank barrel
Procedure 3.
Mass of one stock tank barrel
Procedure 3.
And the gas gravity at reservoir conditions
is
 gR 
R g  4 ,600  o
R  133 ,300  o / Mo
 An approximation for Mo (when not given is)
42.43  STO
5 ,954
Mo  o

API  8.8 1.008   STO
Procedure 3.
For two-stage separators
g 
RSP  gSP  RST  gST
R  RSP
RSP  RST
 RST
For three-stage separators … derive
expressions
Once Gas Specific Gravity is
Known
 Evaluate Tpc and Ppc (previous paper
using K and J and including corrections
for impurities N2, CO2, H2S)
If dew-point pressure is not known
 Use dry-gas z-factor when C7+ < 4%
 Or when wellstream gravity < 0.911
 If pd is known
 if reservoir p is lower than pd evaluate z2phase using equation from SPE 20055 paper
 If reservoir p is greater than pd , evaluate z as
for a dry gas (single-phase)
Correlation of Specific Gravities
for a wet gas
READ - SPE 20055
Ranges of Compositions
Single vs Two-phase z-factor
p/z-2phase v s z-2phase
1.2
z-gas (one phase )
z, z-2phase
z-2phase s
1
0.8
0.6
0
1000
2000
3000
Pre ssure (psia)
4000
5000
Estimates of the Gas in Place (G)
 When p/z = 0  Gp = G
Gp
pi 
p


1

z
zi 
G

From single phase z
From two-phase z
G=
G=
(PETE 323)




644640 MMSCF
679522 MMSCF
5.13328 % difference
Gp 
pi 
p
 1 


z  Exercise:
zi 
G
 these calculations using
verify
information from next slide
Estimates of Reserves
P/z_1phase = -9.359752E-03x + 6.033670E+03
R2 = 9.979569E-01
Gas in Place Prediction
p/z , p/z-2phase (psia)
7000
p/z-2phase
p/z-gas
Linear (p/z-gas)
Linear (p/z-2phase)
6000
5000
4000
3000
2000
P/z_2phase = -8.405311E-03x + 5.711591E+03
R2 = 9.979049E-01
1000
0
0
100000
200000
300000
400000
500000
Gp (MMSCF)
600000
700000
800000