l14 - wet gases
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PETE 310
Lecture # 14
Wet Gas Specific Gravity & Z-factor
(Chapter 7: pages 195-205)
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Learning Objectives
Calculate the specific gravity of a wet gasmixture, given producing GOR (at separator(s)and stock tank and:
compositions liquid and gas from stock tankand separator gas
or, separator compositions (gas & liquid)
or, properties of the separator gas and stockvent 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-factorisotherm.
Calculate values of two-phase z-factor usingRayes etal. correlation (SPE paper).
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Separators
iSTx
)STB/scf(GORandy iST
)STB/scf(GORand
y iSP
iSTx
Wellhead
iSPx
( )( ) ( )
( )( )SToilgas
STgasv
SToilgasSPoil
SPoilgas
SPgas
v
molelbmolelbmolelbf
molelbmolelbmolelb
molelbmolelb
molelb
f
ST
SP
+=
+=
+
=
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Key Points
What matters is the molar ratio of gas to oil
so lets assume one barrel of oil producedMethods to evaluate oil density will be
discussed in Chapter 11 (here it will be
provided)
To convert oAPI to oil density
w
oo
o
o 5.1315.141
API
=
=
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Key Points
The expression [=] means has the units of Forexample
You are responsible for reading the material thatcannot be covered in this lecture
Rework ALL the example problems in the bookProcedure 1 - explained in detail here - is simplerand takes less time to solve than the method
explained in the book
[ ]3o ft
lb=
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Recombination procedure when
separator gas y iSP and tock tankcompositions (x iSTO, yiST) are
known
(Procedure 1.)
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Procedure 1.
Calculate molecular weight of stock tank oil
(STO also referred as STB)
Calculate lb-moles of separator gas produced
per barrel of STO
=
=Nc
1i
wiiwo MxM
[ ] ( )( )
[ ]
volume)molargas(idealmole-b380.7scf/lV
/STOmole-lbmole-scf/lb
scf/STB
V
GOR
idm
gasid
m
SP
=
==
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Procedure 1.
Calculate lb-moles of stock gas vented per
STO
Calculate lb-moles of oil in 1 barrel of stock
tank (need to use molar density)
[ ] ( )( )
[ ]
[ ] /STOmole-lbbblft615.5ftmole-lb
ft
mole-lb
mole-lb/lb
lb/ft
M
oil
3
3oil
3
oil3
wo
oil
=
==
[ ] ( )
( )
[ ] /STOmole-lb
mole-scf/lb
scf/STB
V
GORgasid
m
ST ==
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Procedure 1.
( )
( ) ( )
( )
( )STOmolelbmolelb
STO
molelb
f
molelbmolelbmolelb
STO
molelbmolelbSTOmolelb
f
SToilgas
STgas
v
SToilgasSPoil
SPoilgas
SPgas
v
ST
SP
+
=
+=
+
=
iSTx
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Procedure 1.
Determine reservoir gas composition fromfundamental mole balance
Once reservoir composition is known determine z-factor and specific gravity
( )( )[ ]( )
SPSTSTSP
STST
SPSP
vvSTivSTivSPii
vSTivSTiSPi
vSPivSPii
ffxfyfyz
fxfyx
fxfyz
++=
+=
+=
11
1
1 iSTx
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Example for Procedure 1.
Yi SEP Yi STOX i STO
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Recombination procedure whenseparator gas y iSP and liquid
compositions xiSP
are known
(Procedure 2.)
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Example for Procedure 2.
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Procedure 2.
Additional information given is the separator/stocktank volume ratio as
Use this to convert from scf/STO scf/SeparatorOil
Proceed as in procedure 1.Rework example 7.2 in textbook
)conditionsstandardat(STBbbl
separator)ofP(T,atiloSPbbl
f1xfyzSPSP vSPivSPii
+=
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Recombination procedure whenonly separator gas and stock vent
gas properties are known
(Procedure 3.)
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Procedure 3.
For two-stage separators
For three-stage separators derive
expressions
STSP
gSTSTgSPSP
g
RR
RR
+
+=
STSP
RRR +=
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Procedure 3.
Mass of one stock tank barrel
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Procedure 3.
Moles in one stock tank barrel
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Procedure 3.
And the gas gravity at reservoir conditions
is
An approximation for Mo (when not given is)
oo
oggRM/300,133R
600,4R
+
+=
STO
STO
oo 008.1
43.42
8.8API
954,5
M
==
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Once Gas Specific Gravity is
KnownEvaluate Tpc and Ppc (previous paper
using K and J and including correctionsfor 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 z-2phase using equation from SPE 20055 paper
If reservoir p is greater than pd , evaluate z as
for a dry gas (single-phase)
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Correlation of Specific Gravities
for a wet gas
READ - SPE 20055
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Ranges of Compositions
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Single vs Two-phase z-factor
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Applications Material Balance
Estimate of original gas in place
(OGIP)
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Volumetric Calculations
Oil in place = N
[ ]
bbl
STBLfactorConversionSTB
BS1hAreaN
3
o
wnet
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Generalized Material Balance
Equation
Gas
Injection
WaterDrive
Water
Influx
Gas
Oi l
Water
GENERAL MATERIAL BALANCE EQUATION
(GMBE)
Approximates res ervoiras a t ank w ith g iven
boundaries anduniform T and P.
MB C alculation is avery g rossapproximation of thereservoir performance,but one that is veryuseful in prac tice
OIL+GAS+WATER
PRODUCTION
Gas
Oi lWater
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Stirred Tank Diffusive Process
in Reservoir Simulation
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There are 2 hydrocarbon componentsstock-tank oil and surface-gas.
The surface gas can dissolve into both
the reservoir oil- and gas- phases(accounted for by Rs).
The stock-tank oil (also called surfaceoil) cannot be volatilized into the gasphase.
Conventional Material Balance
Assumptions
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Dry gas reservoirs-
Material balance methods
znRTPV=
Volumetric reservoirs
Non-Volumetric reservoirs
Pore volume filled with gas
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Volumetric gas material
balance
Original pressureat reservoirtemperature
New pressure atreservoirtemperature
Time 1 Time 2
RTz/PVn 22 =RTz/PVn 11=Moles of gas
V = X V = X
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A Simple Mass Balance
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Volumetric gas material
balance
Moles of gas produced = n1-n2
380.7 scf per lb mol of gas in Texas (@60deg F and 14.65 psia)
Volume of gas produced =380.7*(n1-n2) scf
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P/z plot Volumetric reservoir
graphic interpretation
G
G1
z
p
z
p p
i
i
P/z
Gp
Gp=G @ P=0
P/z @original pressure
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Estimates of Gas in Place: Use
the two-phase Z-factor
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