Transcript Document
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 )
xiST
xiSP
xiST
Wellhead
fv SP
lb mole
lb mole
gas
gas SP
lb moleoil SP
lb moleoil SP lb molegas
fv ST
lb mole
lb mole
gas
gas ST
lb moleoil 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
GORSP
scf/STB
lb - molegas /STO
id
scf/lb- mole
Vm
Vmid 380.7scf/lb - mole (ideal gas molar volume)
Procedure 1.
Calculate lb-moles of stock gas vented per
STO
GORST
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/ft3
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 fvST 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
b b l SP oil at (T,P of separator)
b b l 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