Gas condensate reservoirs contain fluids that are intermediate between oil and gas. At initial reservoir conditions, the fluid is gas, but during production liquid forms due to pressure and temperature changes. Most gas condensate reservoirs exist at pressures between 3,000-6,000 psi and temperatures of 200-400°F. Efficient production requires either gas reinjection to maintain pressure above dew points or gas cycling to recover liquids and reinject dry gas. Pressure depletion alone is inefficient as it leaves liquids in place and lowers production rates over time.

1. Gas Condensate Reservoir
Chapter 7

2. Gas Condensate Reservoir
Gas condensate may be thought as being intermediate
Gas condensate may be thought as being intermediate
between oil and gas.
between oil and gas.
At initial condition, gas condensate reservoir fluid is gas but
At initial condition, gas condensate reservoir fluid is gas but
during depletion and at separator conditions, substantial
during depletion and at separator conditions, substantial
amount of liquid is formed.
amount of liquid is formed.
Most gas condensate reservoirs exist in pressure ranges of
Most gas condensate reservoirs exist in pressure ranges of
3000 to 6000psia and temperature ranges of 200
3000 to 6000psia and temperature ranges of 200o
oF to
F to
400
400o
oF
F

3. Molecular Comparison and Other Properties
Molecular Comparison and Other Properties
of Typical Reservoir Fluids.
of Typical Reservoir Fluids.
Component Crude oil Gas condensate Dry gas
C1 53.45 87.01 95.85
C2 6.36 4.39 2.67
C3 4.66 2.29 0.34
C4 3.79 1.08 0.52
C5 2.74 0.83 0.08
C6 3.41 0.60 0.12
C7+ 25.59 3.80 0.42
100.00 100.00 100.00
Mol. Wt C7+ 247 112 157
GOR, scf/STB 1.078 18.200 105.000
Tank-oil gravity, API 34.5 60.8 54.7

4. Phase Diagram of Gas-condensate
Phase Diagram of Gas-condensate
Fluids
Fluids

5. Basic Vapour-liquid Equilibrium
Calculations
There is interest in the amount of fluid in both liquid and gas
There is interest in the amount of fluid in both liquid and gas
phases in gas-condensate reservoirs.
phases in gas-condensate reservoirs.
For every component of the reservoir fluid, the distribution of
For every component of the reservoir fluid, the distribution of
the component between vapour and liquid is expressed by:
the component between vapour and liquid is expressed by:
i
i
i
X
Y
K 
Where, Ki = equilibrium ration of component
Where, Ki = equilibrium ration of component
Yi = mole fraction of component i in the the vapour phase
Yi = mole fraction of component i in the the vapour phase
Xi = mole fraction of component i in the liquid phase
Xi = mole fraction of component i in the liquid phase

6. Basic Vapour-liquid Equilibrium
Calculations Cont’d
For a given volume of gas-condensate reservoir fluid
For a given volume of gas-condensate reservoir fluid
consisting of a mixture of different components.
consisting of a mixture of different components.
If we define:
If we define:
n = total number of moles in mixture
n = total number of moles in mixture
L = total number of moles of liquid
L = total number of moles of liquid
V = total number of moles of vapour
V = total number of moles of vapour
Z
Zi
i = mole fraction of component i in mixture
= mole fraction of component i in mixture
Then,
Then,
Z
Zin
in = moles of component i in total mixture
= moles of component i in total mixture
X
XiL
iL = moles of component i in liquid equilibrium
= moles of component i in liquid equilibrium
Y
YiL
iL = mole of component i in vapour at equilibrium
= mole of component i in vapour at equilibrium

7. Basic Vapour-liquid Equilibrium
Calculations Cont’d
A material balance of the mixture in a given volume gives:
A material balance of the mixture in a given volume gives:
n =L + V
n =L + V
A material balance on the ith component gives:
A material balance on the ith component gives:
Z
Zin
in = X
= XiL
iL + Y
+ YiV
iV
At equilibrium, the mole fraction of the components in both phases must sum to
At equilibrium, the mole fraction of the components in both phases must sum to
unit
unit

X
Xi
i = 1
= 1

Y
Yi
i = 1
= 1
Thus:
Thus:
1
V
K
L
nZ
Y
and
1
VK
L
nX
X
i
i
i
i
i









 Using the equation, the equilibrium composition
Using the equation, the equilibrium composition
gas and liquid phases can be calculated at any
gas and liquid phases can be calculated at any
given pressure.
given pressure.
The calculation procedure is a trial and error
The calculation procedure is a trial and error
process.
process.

8. Material Balance Calculations Of Gas-
condensate Reservoirs
If oil zone is negligible MBE takes the same the same
form as dry gas reservoir under both volumetric and
water drive performance.
The MBE can then be written as:
 
  p
w
g
p
e
gi
g
p
w
e
i
i
i
i
b
p
b
W
B
B
G
W
B
B
G
and
ZT
W
B
W
V
P
T
Z
V
P
T
G
P








Note:
Note:
Equations may be used to
Equations may be used to
calculate gas initially in place, G
calculate gas initially in place, G
Gp is the cumulative reservoir gas
Gp is the cumulative reservoir gas
production (total dry gas and gas
production (total dry gas and gas
equivalent of liquid production)
equivalent of liquid production)
Z must be two-phase gas
Z must be two-phase gas
deviation factor
deviation factor

9. Material Balance Calculations Of Gas-
condensate Reservoirs Cont’d
If we assume the following:
If we assume the following:
i.
i. No water influx into the hydrocarbon reservoir
No water influx into the hydrocarbon reservoir
ii.
ii. No water production
No water production
iii.
iii. No reservoir retrograde liquid production
No reservoir retrograde liquid production
iv.
iv. No compaction of reservoir rock
No compaction of reservoir rock
Then the MBE for gas condensate reservoir (also gas reservoirs) becomes:
Then the MBE for gas condensate reservoir (also gas reservoirs) becomes:








G
G
1
Z
P
Z
P p
i
i
abandonment
abandonment
Cumulative Production, G
Cumulative Production, Gp
p
P/Z
P/Z
G
G
Reserve Estimation
Reserve Estimation

10. Material Balance Calculations Of Gas-
condensate Reservoirs Cont’d
Calculation Procedure:
Calculation Procedure:
Obtain field production data .
Obtain field production data .
Convert the stock tank oil production to an equivalent quantity of
Convert the stock tank oil production to an equivalent quantity of
vapour and add it to measure gas production to obtain G
vapour and add it to measure gas production to obtain Gp
p.
.
Determine the tow-phase compressibility.
Determine the tow-phase compressibility.
Calculate P/Z
Calculate P/Z
Make a graph of P/Z vs. G
Make a graph of P/Z vs. Gp
p
From the graph determine G
From the graph determine G
Calculate gas and oil reserves.
Calculate gas and oil reserves.

11. Vapour Equivalent of Stock Tank Oil
Vapour Equivalent of Stock Tank Oil
The equivalent vapour volume of stock tank oil is developed
The equivalent vapour volume of stock tank oil is developed
from depletion data given in the fluid analysis
from depletion data given in the fluid analysis
It is reported as a function of pressure
It is reported as a function of pressure
Pressure psi
Pressure psi
Vapour
Equivalent
of
STO
Vapour
Equivalent
of
STO
MSCF/bbl
MSCF/bbl

12. Estimation of Well Fluid Gravity
The equation for calculating the condensate well fluid
The equation for calculating the condensate well fluid
gravity, given by Ikoku (1980):
gravity, given by Ikoku (1980):
o
o
g
o
g
g
w
M
G
132800
R
4584G
G
R
G



Where, G
Where, Go
o = the tank oil specific gravity
= the tank oil specific gravity
131.5
API
141.5
G
o
o


R
Rg
g = gas-oil ratio
= gas-oil ratio
G
Gg
g = gas gravity
= gas gravity
Mo = Molecular weight of tank oil
Mo = Molecular weight of tank oil
5.9
-
API
6084
M
o
o 

13. Operating Programs For Gas
Condensate Reservoir
1. Pressure depletion without any form of pressure
maintenance or gas return.
2. The produced fluid passed through a gasoline plant
where liquids are recovered and dry gas return to
reservoir
3. Reservoir produced by pressure depletion to economic
limit.

14. Operating Programs For Gas-Condensate
Reservoir Cont’d
Pressure depletion of a gas
Pressure depletion of a gas
condensate reservoir is very
condensate reservoir is very
inefficient as it
inefficient as it
leaves part of the recoverable
leaves part of the recoverable
liquids in the reservoir, and
liquids in the reservoir, and
lowers gas production rate due
lowers gas production rate due
to liquid build-up in the vicinity
to liquid build-up in the vicinity
of producers.
of producers.
In gas cycling, some portion of
In gas cycling, some portion of
the dry produced gas is re-
the dry produced gas is re-
injected.
injected.
The dry gas picks up some of
The dry gas picks up some of
the liquids from the reservoir
the liquids from the reservoir
and is recovered at the
and is recovered at the
wellhead.
wellhead.
If economically attractive, gas injection to maintain reservoir pressure above
Dew Point is preferred over a gas cycling project.