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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