pe 210-fliud properties

7/24/2019 PE 210-Fliud Properties

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Introduction to Petroleum Engineering

PE 210

FLUID PROPERTIES


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The understanding of fluid properties allows petroleumengineers to design and predict the behavior of ahydrocarbon reservoir more accurately.

Natural Gas

Gases often occur in association with crude oil, either asfree gas cap or in solution. This is called "associated gas".

Natural gas that occurs alone is called "non-associated gas".

Methane (C1) is always the major component and occurswith or without other lightl hydrocarbons, and with orwithout inorganic gases.

If natural gas contains less than 0.1 gallons of liquid/1000ft3it is called "dry gas" and this is generally corresponds to

>90% methane. The opposite is "wet gas".

FLUID PROPERTIES


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Natural Gas

Ideal Gas Law:

Relationships that describe the pressure-volume-temperature

(PVT) behavior of gasses are called equation of state.

What is the ideal gas? Is the one in which the volume occupied

by the molecules is small compared to the total volume, i.e.

Vmolecules= 0.0

All molecular collisions are elastic (no lose in energy)

No attractive or repulsive forces among molecules

Why do we study ideal gasses behavior?

To make the understanding of the behavior of real gasses easier.


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Ideal Gas Law

The simplest gas equation of state is a combination of two fundamental

laws:

Boyles Law: (At constant temperature)

OR

Charles Law: (At constant pressure)

OR

Combining the Boyles law and Charles law results:

Where R is the gas constant equals to 10.73 psia.ft3/(ibm-mole R). Note

that avogadro number = 2.73x1026molecules/mole.

pV 1 tconsPV tan=

TV tconsT

Vtan=

nRTPV =


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Gas Specific Gravity

Specific Gravity, g

Defined as the ratio of the density of a gas at a given temperature and pressure

to the density of air at the same temperature and pressure.

Then the specific gravity, g, of a gas is

RT

pair

97.28=

97.2897.28W

W

air

g

gM

RT

PRT

MP

===


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Real Gas Law

Since the volume of a gas will be less than what the ideal gas volume would be,

the gas is said to be supercompressible. The number, which is a measure of the

amount of the gas deviates from the perfect or ideal behavior, is sometimes

called supercompressibility factor, usually shortened to the compressibility

factor (z-factor).

Note that: The z-factor is obtained from figures

Z-factor at standard condition = 1.0

PandTatgasofmolesnofvolumeIdeal

PandTatgasofmolesnofvolumeActual

V

Vz

i

a ==


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Z Correlation

Most correlations are based on the law of corresponding state which applies to the

hydrocarbon gases as they are closely related chemically. The law states at the same Tr

and Prall hydrocarbons have the same values of Z.

The pseudoreduced temperature and pseudoreduced pressure are defined for pure gasses

as:

Z = f (Pr, Tr); therefore,c

rT

TT = , and

c

rP

PP =

(Pc, Tc): The condition above which liquid and vapor phases of the compound cannot be

distinguished.


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Z Correlation

Natural Gases: are mixture of different hydrocarbon gases

They dont have a True Pc, and Tc.

For mixtures, the pseudoreduced temperature and pseudoreduced pressure need a

modification in which the compositions of the gas are taken into the consideration.

Gas analysis is done at atmospheremole fraction = vol. Fraction (Yi= vi) at atmosphere.

Note that Vol. of 1 mole = 379 SCF for all gases.

MW, Pc, and Tcare mole fraction weighted; therefore,

ci

n

i

i

n

i

ciic PVPYP

==

==11

= =

==n

i

n

i

ciiciic TVTYT

1 1

==

==n

i

ii

n

i

iig MWVMWYMW

11

If gas-condensate is produced with the gas its mole fraction must be included in the

composition (since the condensate is a gas in the reservoir).


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Examples

xample (1): What is the volume of 15 pounds of methane at 100oF and 900 psi?

Solution: Given that: at 100oF, 900 psi, 15 # C, (Mwc= 16.04)

PV = ZnRT

n = 935.0#04.16

#15==

wM

W

Gas is C1Fig. (6): Z (900 psia, 100oF) = 0.92

PV = ZnRT

R=10.73, T=100 + 460

V= 5.73 ft3


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Examples

xample (2): If the gas in Example 1 is pure.

Solution: Fig. 9 can be used:

34.1673

900

)(

900===

crc

rP

P

63.1460116

460100=

++=rT

From Fig. 9 Z = 0.92

V = 5.73 ft3

If it is ideal gas (Z=1):

PV = nRT

V = 6.24 ft3

Diff. in value = 9%


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Examples

Example (3):A gas mixture consists of 16 lbs of methane and 7.5 ibs of ethane.

Calculate its volume at 0oF and 1000 psi?

Solution: PV = ZnRT

nT = nc1+ nc2

moleM

mn

wc

cC 1

16

16

1

11 ===

moleM

mn

wC

c

C25.0

07.30

5.7

2

2

2 ===

nT= 1+ 0.25 = 1.25 moleGas Mixture (Fig. 10) Z (Pr, Tr)Tc= Yc1, Tc1+ Yc2Tc2

Yc1= %8080.025.1

11 ===T

c

n

n

Yc2= %2020.025.1

25.02 ===T

c

n

n

Tc = 0.8 (344) + 0.2 (549) = 385oR

Pc = 0.8 (673) + 0.2 (712) = 680.8 psi

Tr = 2.1385

4600=

+

Pr = 469.18.680

1000=

From (Fig. 10) Z = 0.645


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Examples

xample (4:)Estimate Z for 0.8 SG gas at 1390 psia and 980F?

Solution: Pr= 1.2662

1390==

c

P

P

Tr= 35.1413

46098=

+=

cT

T

Pc, Tcfrom (Fig. 11, Fig. 12)

And from (Fig. 10) Z = 0.72


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Gas Formation Volume Factor, Bg

Relates the actual volume of gas at P and T of the reservoir to the actual volume of gas

at standard conditions, pscand Tscon the surface.

sca

TPa

gV

VB

)

)

Scatgasofvol.actual

TP,atgasofvol.actual ,==

Where pscis 14.7 psia and Tscis 60 F,

pTZTpBsc

scg =

=

SCF

ft

p

ZTBg

3

,02829.0

=

SCF

bbl

p

ZTBg ,00504.0


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Gas Formation Volume Factor, Bg


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Isothermal Compressibility

A measure of change in volume with pressure under isothermal conditions.

Since, Therefore:

Note that the above equation can be solved if we know the slope of z-factor plot.

Viscosity:

The viscosity of natural gas depends on the temperature, pressure and composition

of the gas. It has units of (cp).

Carr, Kobayashi, and Burrows have developed correlation charts for estimating the

viscosity of natural gasses.

P

ZnRTV=

2P

ZnRT

P

Z

P

nRT

P

V

=

PP

ZnRT

P

Z

ZP

ZnRT

P

V 11

=

PP

Z

ZP

V

V

111

=

P

V

Vc

=

1

P

Z

Zpcg

=

11


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Crude Oil

A very complex mixture with hundreds of

thousands of compounds. Predominantly

hydrocarbons but also compounds of nitrogen,sulfur and oxygen. The mixture existed in a

liquid phase in an underground reservoir and

remains a liquid at surface after passingthrough surface separation equipment.


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Appearance: Ranges from black, dark brown,greenish through pale colors to water white.

Density: Most oil is less dense than water. Althoughany appropriate units can be used for density, it ismost commonly given as API gravity (AmericanPetroleum Institute)

Light oil are > 40oAPI (0.83 S.G)

Heavy oil are < 25oAPI (1.00 S.G)

( )5.131.

5.141

60/60=

FoGSAPI

Crude Oil Properties


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Crude Oil Properties

Solution Gas-Oil Ratio, Rso

It is a measure of the solubility of natural gas in crude oil.

Depends on the pressure, the temperature, and the composition of the gas and the

crude oil.

For a particular gas and crude oil at constant temperature, the quantity of solution

gas increases with pressure; and at constant pressure the quantity decreases with

increasing temperature. For any temperature and pressure, the quantity of solution

gas increases as the composition of the gas and crude oil approach each other.

A crude oil is said to be saturated with gas at any pressure and temperature if on a

slight reduction in pressure some gas is released from solution. Conversely, if no gas

is released from solution, the crude oil is said to be undersaturated at that pressure.

Rs: gas solubility: No. of SCF of gas which are in solution in 1 STB at reservoir T, P. Or

it is the volume (in SCF) of gas dissolved in 1 STB of oil , SCF/STB.

producedoilofSTB

SCFRs

producedgasof=

Bubble Point Pressure or Saturation Pressure: It is the pressure of a liquid system at

which gas starts bubbling.


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Solution Gas-Oil Ratio

Bubble point: is the condition at which the first bubble of gas

appears.

Dew Point: is the condition at which the first drop of liquid

condenses. It may have a unit of standard cubic feet of solution gas per

stock tank barrel of oil (SCF/STB).

Values of Rs ranges from 50 to 1400 SCF/STb. For most oil

reservoirs, the value of Rs falls between 200 to 1000 SCF/STB. Rs can be obtained from laboratory measurements or can be

estimated from correlation.


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Solution Gas-Oil Ratio


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Oil Formation Volume Factor, Bo

Relates the actual volume of oil at P and T of the reservoir to the actual

volume of oil at standard conditions, pscand Tscon the surface.

The reservoir volume includes dissolved gas, whereas the surface volume

is essentially dead oil and does not include the released gas.

It has units of reservoir barrels of oil per stock tank barrel of oil.

sco

TPo

oVVB

))

Scatoilofvol.actualTP,atoilofvol.actual ,==


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Oil Formation Volume Factor, Bo


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Viscosity

A measure of internal resistance of fluid to flow.

It has a unit of cp.

Below bubble point, the viscosity decreases with increasing pressure owing to

the thinning effect of gas entering solution, but above the bubble point, the

viscosity increases with increasing pressure.

Dead oil is defined as oil with no dissolved gas.

Live oil is defined as oil with dissolved gas.

Values vary widely from the very low value typical of gas to very high

values. In general, low API gravity oils tend to have higher viscosities.


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Water

Water is the dominant fluid in the subsurface. Thetotal quantity of dissolved salts varies from lowvalues in shallow fresh water to more than 400,000mg/liter (ppm) in some deep waters. Values are

usually reported as "Total Dissolved Solids" (TDS).Sea water has 35,000 mg/liter.

Chemical Composition of Reservoir Waters: Waters

are usually analyzed for Chloride (CI) Sulfate (SO4),bicarbonate (HCO3), Carbonate (CO3), Potassium(K+), Calcium (Ca++), magnesium (Mg++), andsodium (Na+).


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Water Properties

Solution Gas-Water Ratio, Rsw

It is a measure of the solubility of natural gas in water.

McCain developed a correlation for the solution gas-water ratio, Rsw(SCF/STB).

Water Formation volume factor, Bw

Relates the actual volume of water at P and T of the reservoir to the actual volume o

water at standard conditions, pscand Tscon the surface. It has units of bbl/STB.

In most cases, the water formation volume factors will be approximately 1.0.

Viscosity

A measure of internal resistance of fluid to flow. It has a unit of cp.

Pure water has a viscosity of 1.0 cp at standard conditions.


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Two phase Formation volume factor, Bt

Relates the actual volume of Liquid and gas at P and T of the reservoir to the actual

volume of Liquid at standard conditions, pscand Tscon the surface. It has units o

bbl/STB.

psi

FL

P

TgL

tSCV

sVVB

7.14

60

@

@

Re)( +=

Note:

The variables Z, Bg, Rs, Bo, and Bt are obtained either by Lab or from correlationcharts. They are all function of Pressure, Temperature, type of oil (S.Go, API) and type o

gas (S.Gg). Lab is more accurate than the correlation charts, but correlation charts are

cheap and take a short time.


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Phase Behavior of Single Component System: Vaporization of a

Pure Substance at constant Temperature

i f Si C S i i f


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Phase Behavior of Single Component System: Vaporization of a

Pure Substance at constant Pressure

When hydrocarbons are produced from the reservoir to the


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When hydrocarbons are produced from the reservoir to the

surface, both pressure and temperature are reduced. Phase

changes may therefore occur in the Produced fluids.


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Example

Given T=200oF, solve for the unknown variables in the following Table:

P L g G Bo Bg SGg Z Rs Bt

psia cc cc SCF bbl/STB bbl/SCfatSC SCF/STB bbl/STB

i 2950 Li 0 0 1.434 - - - 787.911 1.434

1 2630 621 0 0 1.465 - - - 787.911 1.465

2 2220 570 g2 0.977 1.345 0.0013 0.7 0.868 Rs2 1.821

3 900.144 445 455 0.867 1.05 0.0033 0.8 0.893 96.6 2.13SC,60oF 14.7 423.8 7248.9 0.256 1 0.178 0.85 1.0 0 BtSC


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Examples

( ) bblxLBL scoii 3108226.33.1589878.423434.1* =

==

Li= (3.8326 x 10-3) (158987.3) = 607.74 cc

2. bblxscfscf

bblGBg g

3

222 1027.1)977.0(0013.0* =

==

g2= (1.27x10-3bbl) (158987.3 cc/bbl) = 201. 9 cc

3.STBx

SCF

L

GG

L

GR

sc

sc

sc

s)10666.2(

)256.0867.0(3

3

2

+=

+==

Rs2 = 421.23 SCF/STB

4.cc

cc

L

gLB

sc

scsc

tsc)8.423(

)9.248.78.423( +=

+=

Btsc= 18.1 bbl/STB


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The Five Reservoir Fluids

The behavior of reservoir fluids during production isdetermined by the shape of its phase diagram (phase

behavior).

There are five different reservoir fluids:

black oil, volatile oil, retrograde gas, wet gas, and dry gas.

Each type is defined by reference to the shape of its phasediagram

Why do we need the fluid type:

- Type & size of surface equipment

- Estimating oil & gas in place

- Plan of depletion & secondary recovery techniques.


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Black Oil

At point 2, the oil is at its bubble-

point and is said to be saturated.

Rules of thumb:

Initial producing gas-oilratio of or < 2000 scf/stb.

The gravity of stock tank

liquid < 45 API.

The color of the stock-

tank liquid: very dark,often black, sometimes

with greenish cast, or

brown.


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Volatile Oil

Rules of thumb:

Initial producing gas-oil ratio: 2000 3300scf/stb.

The gravity of stocktank liquid > 45 API.

The color of the

stock-tank liquid:usually brown, orange,sometimes green.


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Retrograde Gas

Rules of thumb:

Initial producing gas-oilratio > 3300 scf/stb.

The gravity of stocktank liquid: 40 60 API.

The color of the stock-

tank liquid: brown,orange, greenish, orwater-white.


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Wet Gas

Rules of thumb:

Initial producing gas-

oil ratio > 50,000

scf/stb.

The gravity of stock

tank liquid: 40 60

API.

The color of the

stock-tank liquid:

water-white.


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Dry Gas

Rules of thumb:

Initial producing

gas-oil ratio: no

liquid production.

The gravity of

stock tank liquid:

Not applicable.

The color of the

stock-tank liquid:

colorless.

pe 210-fliud properties

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