tech reservoir pete310 wet gases sp gr zfact

8/11/2019 Tech Reservoir Pete310 Wet Gases Sp Gr Zfact

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PETE 310

Lecture # 14

Wet GasSpecific 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 using

Rayes etal. correlation (SPE paper).


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Separators

iSTx

)STB/scf(GORand

yiST

)STB/scf(GORand

yiSP

iSTx

Wellhead

iSPx

SToilgasSTgas

v

SToilgasSPoil

SPoilgas

SPgas

v

molelbmolelb

molelbf

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 produced

Methods to evaluate oil density will be

discussed in Chapter 11 (here it will beprovided)

To convert oAPI to oil density

w

o

o

o

o5.131

5.141API


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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 book Procedure 1 - explained in detail here - is simpler

and takes less time to solve than the methodexplained in the book

3o ftlb


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Recombination procedure when

separator gas yiSP and tock tank

compositions (xiSTO, yiST) areknown

(Procedure 1.)


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Procedure 1.

Calculate molecular weight of stock tank

liquid

Calculate lb-moles of separator gas produced

per barrel of STO from separator

Nc

1iwiiwo MxM

volume)molargas(idealmole-b380.7scf/lV

/STOmole-lbmole-scf/lb

scf/STB

V

GOR

id

m

gasid

m

SP


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Procedure 1.

Calculate lb-moles of stock gas vented per

STO

Calculate moles of oil in 1 barrel of stock tank

(need to use molar density)

/STOmole-lb

bbl

ft615.5

ft

mole-lb

ft

mole-lb

mole-lb/lb

lb/ft

M

oil

3

3

oil

3

oil3

wo

oil

/STOmole-lbmole-scf/lb

scf/STBV

GORgasid

m

ST


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Procedure 1.

STO

molelbmolelbSTO

molelb

f

molelbmolelbmolelb

STO

molelbmolelbSTO

molelb

f

SToilgas

STgas

v

SToilgasSPoil

SPoilgas

SPgas

v

ST

SP

iST

x


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Procedure 1.

Determine reservoir gas composition fromfundamental mole balance

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

STSTSTSP

STST

SPSP

vvSTivSTivSPii

vSTivSTiSPi

vSPivSPii

f1f1xfyfyz

f1xfyx

f1xfyz

iST

x


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Example for Procedure 1.

Yi SEP Yi STOX i STO


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separator gas yiSP 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/ST

Proceed as in procedure 1.

Rework example 7.2 in textbook

)conditionsstandardat(STBbbl

separator)ofP(T,atiloSPbbl

f1xfyzSPSP vSPivSPii


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only 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

gRR

RR

STSP RRR


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Procedure 3.

Moles in one stock tank barrel


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Procedure 3.

Mass of 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

og

gRM/300,133R

600,4R

STO

STO

oo 008.1

43.42

8.8API

954,5M


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Procedure 3.

For two-stage separators

For three-stage separators deriveexpressions

STSP

gSTSTgSPSP

g RR

RR

STSP

RRR


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Once Gas Specific Gravity is

Known

Evaluate Tpc and Ppc (previous paperusing K and J and including correctionsfor impurities N2, CO2, H2S)

If dew-point pressure is not knownUse dry-gas z-factor when C7+ < 4%

Or when wellstream gravity < 0.911

If pdis known

if reservoir p is lower than pdevaluate z-2phase using equation from SPE 20055 paper

If reservoir p is greater than pd , evaluate z asfor 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

p/z-2phase vs z-2phase

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000

Pressure (psia)

z,

z-2phase

z-gas (one phase)

z-2phases


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Estimates of the Gas in Place (G)

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

Exercise: verify these calculations using

information from next slide

G

G1

z

p

z

p p

i

i

From single phase z G = 644640 MMSCF

From two-phase z G = 679522 MMSCF

5.13328 % difference


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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-2phase

(p

sia)

p/z-2phase

p/z-gas

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

tech reservoir pete310 wet gases sp gr zfact

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