Properties of Dry Gas

CHAPTER 5 5-3 -3 Properties of Dry Gas

1

At the end of the session, students should be

´

´

Describe the properties of Dry gas commonly used by the petroleum engineer. Estimate the values of the properties using normally available information about the dry gas. 2

1

´

Dry gases are the easiest to deal with as it moves from the reservoir to the surface.

´

The gas compositions at the surface and therefore, the specific gravity would also be the same for both. 3

´

Dry gases are the easiest to deal with as it moves from the reservoir to the surface.

´

The gas compositions at the surface and therefore, the specific gravity would also be the same for both. 4

2

´

The gas formation volume factor (Bg) is defined as the volume of the gas at the reservoir conditions required to conditions.

´

´

´ ´

The reciprocal of the Bg sometimes is called gas expansion factor. The usual units for the B are: Reservoir cubic feet per standard cubic feet, res. cu ft/ scf. Reservoir barrels per standard cubic feet, res. bbl/scf.

5

What are the standard conditions???

Volu me of th e gas at the reservoir P&T Volu me of the same mass of gas at the s ur ac e

B g

V R V SC

6

3

B g =

V R V SC

The volume of n moles of gas at reservoir condition

V R =

z R nRT R

V SC =

P R

z sc nRT sc Psc

The volume of n moles of gas at standard condition Thus, the formation volume factor is

z R n B

g

=

z sc

P R nRT

B g = 0 . 0282

R

sc

=

P sc

B g = 0 . 00502

Psc = 14.7 psia, Tsc= 520 R, z = 1

T cu . t P

scf

zT res .bbl P

scf 7

g B

Pressure 8

4

´

Calculate the formation volume factor o a dr as with a s eci ic ravit o 0.818 at reservoir temperature of 220F and pressure of 2100 psig.

First, estimate the P pc and T pc econ . a cu a e

e

pr

an

pr

an ge

e z ac or.

Third, Calculate the Bg using the following equation: B g = 0 . 00502

zT res .bbl P

scf

9

T =220F P = 2100 psig First, from this figure and at 0.818 gravity Ppc = 647psia . pc =

Second calculate the Tpr and Ppr

TPR =

T TPC

T PR =

PPR = Resource: McCain textbook pg. 119

PPR =

(220 + 460) R 406 R

P PPC

= 1.68

(2100+14.7) psia 647 psia

= 3.27 10

5

Third, Find z value for the dry gas using this chart Ppr = 3.27 and Tpr = 1.68

z = 0.855

Resource: McCain textbook pg. 112

11

´

Calculate the formation volume factor of a dry gas with a specific gravity of 0.818 at reservoir temperature of 220F an pressure o ps . Once Z factor is estimated, then you can use the following equation for calculating the gas formation volume factor

B g = 0 . 00502

B g = 0 . 00502

zT res .bbl P

( 0 . 855 )( 220 + 460 ) 2100 + 14 . 7

scf

= 0 . 00138

res .bbl scf 12

6

´

The coefficient of isothermal compressibility (Cg) is defined as the fractional change of volume as pressure is changed at a constant temperature.

´

The coefficient normally is referred to as gas compressibility (Cg)

´

Compressibility factor ,z, is not the .

´

g

C , y t i l i b i s s e r p m o c s G

The unit for Cg is psi-1

cg = −

´

´

Reservoir pressure

1 ⎛ ∂V ⎞ 1 ⎛ ∂ Vm ⎞ or = − ⎜ ⎟ ⎜ ⎟ V ⎝ ∂P ⎠ Vm ⎝ ∂ P ⎠

13

Viscosity is a measure of the resistance to flow exerted by a fluid This is called dynamic viscosity and has units of centipoise = g mass / 100 sec cm

´

Kinematic viscosity is viscosity / density, units are in centistokes = centipoise /g/cc = cm2 /100 sec

14

7

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Gas viscosity decreases as reservoir pressure decreases. The molecules are a apart at low pressure and move past each other more easily. ow pressures an ncrease n empera ure ncreases gas v scos y w ereas at high pressure gas viscosity decreases as the temperature increases ) p c ( y t i s c s i V

T increasing

Pressure

15

The viscosity of the gas mixtures can be calculated using the following Equation. This equation is only used when the gas composition and v scos y or eac are nown. μ g

=

∑ μ y M ∑ y M gj

j

j

= =

μ gj

y j = M

j

1/ 2 j

1/ 2 j

Viscosity of gas mixtures Viscosity of component j in the gas mixtures Mole fraction of component j in the gas mixtures

=

Molecular weight of component j in the gas mixtures 16

8

Calculate the viscosity of the gas mixture given below at 200F and a pressure of one atmospheric absolute. Mw 16.04 30.07 44.10 58.12

1/ 2

=

g

´ ´ ´

∑

g

1/ 2

y j M j

= 0.0125 cp

Read Molecular Weights Read Viscosities this figure Apply formula…


17

When the compositions of the mixture are not available, the viscosity of the mixture can be estimated as a function of the gas specific gravity with the aid of the following chart.


18

9

=

ratio

×

make sure you check the specific gravity range

atm

where

atm

Can be obtained as presented

ratio Can be obtained as

a function of Ppr and Tpr with the aid of this chart. 19

20

10

Properties of Dry Gas

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