Phase Behaviour dan Characteristics Gas

Single-component system

Two-component system

Two-component system

Multi-component system

Temperature

Pre

ssu

re liquid

gas

C

Cricondenbar

Cricondentherm

Definitions:

Cricondenbar: the pressure above which two phases can no longer exist.

Cricondentherm: the temperature above which two phases do not exist

Additional points:

Phase Diagram (Basic Definitions)

Phase Diagram (Basic Definitions)

Temperature

Pre

ssu

re

liquid

gas

100% liquid(0% gas)

80%60%

40%

20%

C

0% liquid(100% gas)

Pressure Temperature Plane

Definitions:

1. Bubble-point line: the point where the first bubble is formed during pressure decrease at constant temperature.

2. Dew-point line: the point where the first liquid drop is formed during pressure increase at constant temperature.

Note: Pure-component system can be regarded as a special case of two-component system where two-phase region shrinks to form a line.

Critical point: the point where the bubble-point line meets the dew-point line

Temperature

Pre

ssu

re

Single phaseliquid

gas

CB

ub

ble

-po

int

(pre

ssu

re)

line

Dew

-po

int

(pre

ssu

re)

line

Single phaseGas

Phase Diagram (Basic Concepts)

Temperature

Pre

ssu

re liquid

gas

C

“Retrograde condensation”

Liq

uid

-mixtu

re-gas

Path A Path B Definitions:

Retrograde condensation: phenomenon that the dew point line is crossed (i.e., from gas phase to liquid) as pressure decreases rather than increases.

Because this is the reverse of normal behaviour, it is called “retro”.

Basic definitions (cont.)

Reservoir Classification Oil reservoir

In general Tres<Tc of reservoir fluid

Gas reservoir In general,

Tres>Tc of reservoir fluid (hydrocarbon systems)

Oil Reservoir

Under-saturated oil reservoir initial reservoir

pressure, pi > the bubble-point pressure, pb of the reservoir fluid

Saturated oil reservoir pi = pb

Gas-cap reservoir or two phase reservoir pi < pb

Note The appropriate quality

line gives the ratio of volume of liquid (oil) to volume of gas

Gas ReservoirDry gas reservoir

initial reservoir temperature higher than cricondentherm temperature (light components)

even at low pressure (separator) and temperature, fluid is 100% gas

Wet gas reservoir initial reservoir temperature

higher than cricondentherm temperature

But even at low pressure (separator) and temperature, some gas condensate to liquid

Retrograde gas-condensate reservoir Reservoir temperature lies

between Tc and Tcri (Tc<Tr<Tcr)

Near critical gas-condensate Reservoir temperature is

nearly equal to critical temperature of fluid (Tr ~Tc)

Dry-gas reservoir

Wet-gas reservoir

Retrograde-gas condensate reservoir

Volatile-oil reservoir

Black-oil reservoir

Typical Hydrocarbon Mixture Compositions (mol %)

C1 88.7

C2

C3

i-C4

n-C4i-C5

n-C5

C6

C7+

6.0

3.0

0.5

0.8

0.3

0.3

0.2

0.2

96.303.00

0.40

0.07

0.10

0.02

0.02

0.02

0.00

72.710.0

6.0

1.0

1.5

0.8

1.0

2.0

5.0

66.79.0

6.0

0.8

2.5

0.8

1.2

2.0

11.0

52.65.0

3.5

0.7

1.1

0.4

0.4

0.9

27.9

DryGas

WetGas

RetrogradeGas Cond.

VolatileOil

BlackOil

Component

plus inorganics: N2, CO2, H2O, H2S

Field Identification

BlackOil

VolatileOil

RetrogradeGas

WetGas

DryGas

InitialProducingGas/LiquidRatio, scf/STB

<1750 1750 to3200

> 3200 > 15,000* 100,000*

Initial Stock-Tank LiquidGravity, API

< 45 > 40 > 40 Up to 70 NoLiquid

Color of Stock-Tank Liquid

Dark Colored LightlyColored

WaterWhite

NoLiquid

*For Engineering Purposes

Karakteristik Gas Ideal Gas Law Pada tekanan rendah, gas mengikuti persamaan Gas Ideal:

PV = nRT

Dimana: R = Universal gas constant , 10,732 psia cuft/lb-mole oR n = number of moles of the gas = m/M m = mass of gas; lb M = Molecular weight of gas lbm/lb-mol Standard conditions are generally defined as 14.7 psia and 60oF. 1 mole of any ideal gas at standard conditions occupies a volume of 379 Cu. ft. or 22.4 litres

Real Gas Persamaan gas ideal umumnya hanya berlaku untuk gas yang mendekati kondisi atmosfir. Gas di reservoir terdiri dari berbagai macam komposisi gas dan tekanan serta temperatur

tinggi; sehingga persamaan gas ideal tidak berlaku lagi. Persamaan gas nyata merupakan koreksi terhadap persamaan gas ideal

PV = z n RT.

Dimana

Sifat Fisik Gas Alam

Gas alam merupakan campuran gas dimana unsur utamanya adalah metana; terdiri dari gas hidrokarbon dan impurities.

Impurities adalah gas yang bukan hidrokarbon seperti : CO2, H2S, N2 dsb

Sifat fisik gas alam antara lain adalah: - Berat Molekul - Densitas - Specific Gravity - Faktor Deviasi gas - Viscositas Gas - Compressibilitas gas - Gas Formation Volume Factor dan Expansion Factor - Water Vapor Content - Two phase System

- API Gravity - Gas Gravity - Z factor

Berat Molekul gas

Berat Molekul gas dapat ditentukan berdasarkan persamaan:

Ma = Berat molekul gas campuran Yi = Fraksi mol gas – i Mi = Berat molekul gas –i

Contoh:

Berat molekul gas diatas adalah 17.083 lbm/lb-mol.

Densitas dan Specific Gravity Gas

Berdasarkan persamaan gas nyata : PV = n Z RT ; dimana n = m/M

Jadi PV = m ZRT/M Sedangkan densitas adalah m/V; sehingga dari persamaan diatas :

Faktor Deviasi Gas, Z

Pengukuran Z1. Di laboratorium 2. Grafik korelasi dan persamaan

Pengukuran Z di laboratorium

Penentuan Z berdasarkan Grafik Korelasi / Persamaan

Penentuan Z berdasarkan Grafik Korelasi / Persamaan memerlukan faktor pseudo reduced pressure (Ppr) dan pseudo reduced temperature (Tpr) .

Ppr dan Tpr didefinisikan sebagai

Sedang Tpc dan Ppc didefinisikan sebagai:

Apabila komposisi gas tidak diketahui, Tpc dan Ppc dapat ditentukan dari grafik dibawah ini:

Apabila kadar impurities > 5 %

Contoh

Compressibilitas

Viskositas Gas

Korelasi yang umum digunakan adalah korelasi Carr, Kobayashi

dan Burrows.

Contoh:

Tentukanlah viskositas gas reservoir pada kondisi P = 2500 psia dan T = 180 oF.

Gas Formation Volume Factor (Bg)

The gas formation volume factor (Bg) is defined as the volume of gas in the reservoir divided by the volume of gas at standard conditions.

Bg = (reservoir gas volume)/(standard conditions gas volume).

Gas Formation Volume Factor (Bg)

Jika Tekanan reservoir = 2900 psia dan Temperatur reservoir = 190 oF; tentukanlah: SG, Z, Viskositas Gas, Cg dan Bg

Komponen Fraksi MolC1 0,784C2 0,028C3 0,007i-C4 0,0008n-C4 0,0005i-C5 0,0008n-C5 0,0003C6 0,0006N2 0,005CO2 0,021H2S 0,152

Dari suatu reservoir gas diketahui SG gas = 0.7 dan terdapat impurities H2S = 5 % mole dan CO2 = 12 % mole.

Jika Tekanan reservoir = 2900 psia dan Temperatur reservoir = 190 oF; tentukanlah:

SG, Z, Viskositas Gas, Cg dan Bg

Real Gas

PV = Z n RT

Persamaan Van der Walls

Standard Units of Defined Calorific ValuesBarrel of Oil Equivalent (b.o.e.) : a hypothetical “barrel of oil” with an average heat content of 5.8 x 106 Btu gross. Ton of Oil Equivalent (t.o.e.) : a hypothetical “ton of oil” with an average heat content of 43 x 106 Btu gross. Metric Tonne Coal Equivalent (m.t.c.e.) : a hypothetical “metric tonne of coal” with an average heat content of 27.337 x 106 Btu gross, i.e. ( 12,400 Btu/lb x 2204.62 ).

Std. Fuel Units ft3 Natural Gas m3 Natural Gas

1 b.o.e. 5,800 155.50

1 t.o.e. 43,000 1152.82

1 m.t.c.e. 27,337 732.90

1 ton fuel oil equiv. 41,400 1109.92

Natural Gas Equivalents of Various Fuels