Chapter 6: Properties Of Dry Gas Flashcards
What order (i.e. 10^[]) should compressibility be reported?
What are the units?
10^[-6] sips
Or
1 microsip
What is the equation for formation vol factor of a gas?
Define each term
Bg = VR = z T Psc
___ = ________
Vsc zsc Tsc P
VR = vol at res conditions Vsc = vol at standard conditions
What are the 5 steps to calculate the coefficient of isothermal compressibility of the dry gas at T & P?
- Determine the pseudo critical properties (T/Tc and P/Pc)
- Calculate the pseudo reduced properties (T/Tpr and P/Ppr)
- Determine z and (partial z/partial Ppr) at constant Tpr
(slope of tangent line) - Calculate Cpr = (1/Ppr) - (1/z) (slope of tangent)
- Calculate Cg = (Cpr/Ppc)
What equation would you use to calculate the vol occupied by 1 lb-mol of natural gas at std conditions?
(Specify the values for standard conditions)
Assuming ideal behavior
Vm = RTsc
___
Psc
Vm = molar vol Tsc = 60 F Psc = 14.65 psia
For a DRY gas, relate the composition & specific gravity in the reservoir to that on the surface.
Composition of the gas in the reservoir is EQUAL to the composition of the gas at the surface
Specific gravity of the reservoir gas is EQUAL to the specific gravity at the surface
How is the gas formation vol factor (Bg) defined?
What are the typical units?
The vol of gas at reservoir conditions required to produce 1 standard cubic foot of gas at the surface
Typical units are reservoir barrels of gas per standard cubic foot
(res bbl/scf)
When is the Joule-Thomson effect valid?
What values do you need to solve the equation?
What sign (+/-) are the values?
Its valid when the change in pressure is adiabatic & small
Vm, T, z, Cp
They are always positive
The temperature change in the Joule-Thomson effect is directly related to what?
The attraction of the molecules for each other
What is the Joule-Thomson effect?
The temperature change as pressure is reduced when a flowing stream of gas passes through a throttle*
- throttle = a valve, choke, or perforation in casing
How do you convert gross heating value (dry) into gross heating value (wet)?
Gross: Lc(wet) = (1-0.0175) * Lc(dry) + 0.9
0.9 accounts for the heat released (BTU/scf) during condensation of the water vapor which was in the gas prior to combustion
How do you convert net heating value (dry) into net heating value (wet)?
Net: Lc(wet) = (1- 0.0175) * Lc(dry)
= (1 - 0.0175) * sum (y * Lc)
Where 0.0175 is the mole fraction of water vapor in the gas when saturated at standard conditions
How do you calculate the heating value of an IDEAL gas? [dry basis]
What type of heating value(s) does this calculate?
Lc(ideal,dry) = sum(y * Lc)
Calculates gross & net heating values
How do you convert the gross or net heating values from IDEAL gas to REAL gas?
Lc = Lc(ideal) = Sum(y * Lc)
________ ___________
Z Z
What is the difference between net and gross heating values?
The heat of vaporization of the water of combustion;
- The water of combustion remains vapor @ standard conditions
What does the net heating value mean?
The heat produced in:
- complete combustion
- under constant pressure
- with the combustion products cooled to standard conditions
- the water in the combustion products remains in the vapor state
What does gross heating value mean?
The heat produced in:
- complete combustion
- under constant pressure
- with the combustion products cooled to standard conditions
- the water in the combustion products condensed to the liquid state
What does dry heating value mean?
The gas contains no water vapor
What does wet heating value mean?
What is the approximate volume %?
The gas is saturated with water vapor
- about 1.75 volume %
How does gas viscosity change when:
a) reservoir pressure decreases
b) temperature increases (at LOW pressure)
c) temperature increases (at HIGH pressure)
a) viscosity decreases as res pressure decreases
b) viscosity increases as temp increases at LOW pressures
c) viscosity decreases as temp increases at HIGH pressures
What is the equation used to determine the volume at std conditions in:
a) words
b) an equation
a) the vol of the same N # of moles (in the res) at standard T & P
b) Vsc = Zsc N R Tsc
____________ , Zsc =1 , Tsc = 520 R , Psc = 14.65 Pisa
Psc
What is the equation used to determine the volume at reservoir conditions in:
a) words
b) an equation
a) the volume of N # of moles of a gas at reservoir conditions at reservoir T & P
b) Vres = z N R T
________
P
What is the final equation for the formation volume factor for a gas after standard conditions are substituted, in:
a) cu ft / scf
b) res bbl / scf
a) Bg = 0.0282 ( z T / P ) cu ft / scf
b) Bg = 0.00502 ( z T / P ) res bbl / scf
- T is in Rankin
- P is in psia
What is the definition of the coefficient of isothermal compressibility in:
a) words
b) an equation, with units
a) the fractional change of volume as pressure is changed at constant temp
b) Cg = - (1/V) (partial V / partial P) , 10^[-6] sips
What conditions of the gas do the following refer to:
a) wet & dry
b) gross & net
a) wet & dry => conditions prior to combustion
b) gross & net => conditions of the water of combustion after
burning takes place
What is the heating value of a gas?
What are the units?
The heating value is the quantity of heat produced when the gas is burned completely to CO2 and H2O
Expressed in BTU/scf
How do you calculate the specific gravity of a gas mixture?
Ma = sum(y * M)
(y * M) = y (mol fraction) * M (molecular weight)
Specific gravity = Ma / 29
What effect does the presence of non-hydrocarbon gases ( H2S, N2, CO2) have on a gas mixture’s viscosity?
Non-hydrocarbon gases increase the viscosity of the gas mixture
How do you estimate the z factor in the formation volume equation for the following:
a) experimental value unavailable, but known composition
b) only the specific gravity of the gas is known
a) compute pseudo reduced properties, find z from Compressibility
chart
b) find pseudo critical properties from Pseudo critical Properties of
Natural Gases chart, then use Compressibility chart to find z
factor
What is the gas shrinkage factor?
What are the units?
The reciprocal of the formation volume factor
bo = (1 / Bg) , scf / res bbl
