PETE 310

PETE 310PETE 310

Lecture # 14Lecture # 14

Wet Gas – Specific Gravity & Z-factorWet Gas – Specific Gravity & Z-factor

(Chapter 7: pages 195-205)(Chapter 7: pages 195-205)

Learning ObjectivesLearning Objectives Calculate the specific gravity of a wet gas mixture, given Calculate the specific gravity of a wet gas mixture, given

producing GOR (at separator(s) and stock tank and:producing GOR (at separator(s) and stock tank and:compositions liquid and gas from stock tank and compositions liquid and gas from stock tank and

separator gasseparator gasor, separator compositions (gas & liquid)or, separator compositions (gas & liquid)or, properties of the separator gas and stock vent gasor, properties of the separator gas and stock vent gas

Define the two-phase z-factor and understand the uses Define the two-phase z-factor and understand the uses of this in reservoir engineeringof this in reservoir engineering

Explain the shape of a typical two-phase z-factor Explain the shape of a typical two-phase z-factor isotherm. isotherm.

Calculate values of two-phase z-factor using Rayes etal. Calculate values of two-phase z-factor using Rayes etal. correlation (SPE paper).correlation (SPE paper).

SeparatorsSeparators

iSTx

)STB/scf( GOR and

y iST

)STB/scf( GOR and

y iSP

iSTx

Wellhead

iSPx

SToilgas

STgasv

SToilgasSPoil

SPoilgas

SPgasv

molelbmolelb

molelbf

molelbmolelbmolelb

molelbmolelb

molelbf

ST

SP

Key Points Key Points

What matters is the What matters is the molar ratiomolar ratio of gas to oil of gas to oil so let’s assume one barrel of oil produced so let’s assume one barrel of oil produced

Methods to evaluate oil density will be Methods to evaluate oil density will be discussed in Chapter 11 (here it will be discussed in Chapter 11 (here it will be provided)provided)

To convert To convert ooAPI to oil densityAPI to oil density

w

oo

o

o 5.1315.141

API

Key Points Key Points

The expression [=] means “has the units of…” For The expression [=] means “has the units of…” For example example

You are responsible for reading the material that You are responsible for reading the material that cannot be covered in this lecturecannot be covered in this lecture

Rework ALL the example problems in the bookRework ALL the example problems in the book

Procedure 1 - explained in detail here - is simpler Procedure 1 - explained in detail here - is simpler and takes less time to solve than the method and takes less time to solve than the method explained in the bookexplained in the book

3o ft

lb

Recombination procedure when Recombination procedure when separator gas yseparator gas yiSP iSP and tock tank and tock tank

compositions (xcompositions (xiSTOiSTO, yi, yiSTST) are ) are

knownknown

(Procedure 1.)(Procedure 1.)

Procedure 1.Procedure 1.

Calculate molecular weight of stock tank Calculate molecular weight of stock tank liquid liquid

Calculate lb-moles of separator gas produced Calculate lb-moles of separator gas produced per barrel of STO from separatorper barrel of STO from separator

Nc

1iwiiwo MxM

volume) molargas (ideal mole-b380.7scf/lV

/STOmole-lb mole-scf/lb

scf/STB

V

GOR

idm

gasidm

SP


Calculate lb-moles of stock gas vented per Calculate lb-moles of stock gas vented per STOSTO

Calculate moles of oil in 1 barrel of stock tank Calculate moles of oil in 1 barrel of stock tank (need to use molar density)(need to use molar density)

/STOmole-lb bbl

ft615.5

ft

mole-lb

ft

mole-lb

mole-lb/lb

lb/ft

M

oil

3

3oil

3oil

3

wo

oil

/STOmole-lb

mole-scf/lb

scf/STB

V

GORgasid

m

ST


STO

molelbmolelbSTO

molelb

f

molelbmolelbmolelb

STO

molelbmolelbSTO

molelb

f

SToilgas

STgas

v

SToilgasSPoil

SPoilgas

SPgas

v

ST

SP

iSTx


Determine reservoir gas composition from Determine reservoir gas composition from fundamental mole balancefundamental mole balance

Once reservoir composition is known determine z-Once reservoir composition is known determine z-factor and specific gravityfactor and specific gravity

STSTSTSP

STST

SPSP

vvSTivSTivSPii

vSTivSTiSPi

vSPivSPii

f1f1xfyfyz

f1xfyx

f1xfyz

iSTx

Example for Procedure 1.Example for Procedure 1.

Yi SEP Yi STOX i STO

Recombination procedure when Recombination procedure when separator gas yseparator gas yiSP iSP and liquid and liquid

compositions xcompositions xiSP iSP are knownare known


Example for Procedure 2.Example for Procedure 2.

Procedure 2. Procedure 2.

Additional information given is the separator/stock Additional information given is the separator/stock tank volume ratio astank volume ratio as

Use this to convert from scf/STO Use this to convert from scf/STO scf/ST scf/ST

Proceed as in procedure 1.Proceed as in procedure 1.

Rework example 7.2 in textbookRework example 7.2 in textbook

)conditions standard at( STB bbl

separator) of P(T, at ilo SP bbl

f1xfyzSPSP vSPivSPii

Recombination procedure when Recombination procedure when only separator gas and stock vent only separator gas and stock vent

gas properties are knowngas properties are known



For two-stage separatorsFor two-stage separators

For three-stage separators … derive For three-stage separators … derive expressionsexpressions

STSP

gSTSTgSPSPg RR

RR

STSP RRR


Moles in one stock tank barrelMoles in one stock tank barrel


Mass of one stock tank barrelMass of one stock tank barrel


And the gas gravity at reservoir conditions And the gas gravity at reservoir conditions isis

An approximation for Mo (when not given is)An approximation for Mo (when not given is)

oo

oggR M/300,133R

600,4R

STO

STOoo 008.1

43.42

8.8API

954,5M


For two-stage separatorsFor two-stage separators

For three-stage separators … derive For three-stage separators … derive expressionsexpressions

STSP

gSTSTgSPSPg RR

RR

STSP RRR

Once Gas Specific Gravity is Once Gas Specific Gravity is KnownKnown

Evaluate Tpc and Ppc (previous paper Evaluate Tpc and Ppc (previous paper using K and J and including corrections using K and J and including corrections for impurities Nfor impurities N22, CO, CO22, H, H22S)S)

If dew-point pressure is not knownIf dew-point pressure is not knownUse dry-gas z-factor when C7+ < 4%Use dry-gas z-factor when C7+ < 4%Or when wellstream gravity < 0.911Or when wellstream gravity < 0.911

If pIf pdd is known is known if reservoir p is lower than pif reservoir p is lower than pdd evaluate z- evaluate z-

2phase using equation from SPE 20055 paper2phase using equation from SPE 20055 paper If reservoir p is greater than pIf reservoir p is greater than pd d , evaluate z as , evaluate z as

for a dry gas (single-phase)for a dry gas (single-phase)

Correlation of Specific GravitiesCorrelation of Specific Gravitiesfor a wet gasfor a wet gas

READ - SPE 20055READ - SPE 20055

Ranges of CompositionsRanges of Compositions

Single vs Two-phase z-factorSingle vs Two-phase z-factor

p/z-2phase vs z-2phase

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000

Pressure (psia)

z,

z-2

ph

as

e

z-gas (one phase)

z-2phases

Estimates of the Gas in Place (G)Estimates of the Gas in Place (G)

When p/z = 0 When p/z = 0 G Gpp = G = G (PETE 323)(PETE 323)

Exercise: verify these calculations using Exercise: verify these calculations using information from next slideinformation from next slide

G

G1

z

p

z

p p

i

i

From single phase z G = 644640 MMSCFFrom two-phase z G = 679522 MMSCF

5.13328 % difference

G

G1

z

p

z

p p

i

i

Estimates of Reserves Estimates of Reserves

Gas in Place Prediction

P/z_1phase = -9.359752E-03x + 6.033670E+03

R2 = 9.979569E-01

P/z_2phase = -8.405311E-03x + 5.711591E+03

R2 = 9.979049E-01

0

1000

2000

3000

4000

5000

6000

7000

0 100000 200000 300000 400000 500000 600000 700000 800000

Gp (MMSCF)

p/z

, p

/z-2

ph

as

e (

ps

ia)

p/z-2phasep/z-gas

Linear (p/z-gas)Linear (p/z-2phase)

PETE 310

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