TOPIC 1: Gas Hydrates
43
©H
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ATT
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NIVERSITY B41OA December 2018 v3
Worked Example 1: Solution
When N
2
and CO
2
are excluded the molar ratio,
m
F
between the non-hydrate
formers (
nh
) and the hydrate formers (
h
) becomes:
1061
.
0
73
.
87
31
.
9
=
=
=
h
nh
m
f
f
F
The specific gravity of the hydrate formers
γ
is calculated by:
Using equation (1.3) the gas gravity of the hydrate forming hydrocarbons then
become
6766
.
0
966
.
28
598
.
19
=
=
=
a
h
M
M
γ
The values,
55
.
280
=
T
K and
1061
.
0
=
m
F
, are inserted into the correlation,
equation (1.2), which results in a hydrate dissociation pressure for the
hydrocarbon fluid alone:
1630
=
hf
P
kPa
To take into
account the effect of CO
2
, the mole fraction of CO
2
in
the gas
condensate is calculated on a N
2
free base (there is no order of preference to
include the effect of N
2
and CO
2
on the hydrate free zone):
(
)
0239
.
0
58
.
0
100
38
.
2
2
CO
=
=
=
f
Getting the constants for carbon dioxide from Table 3 and substitute them into
equation (1.7) below to find the alpha parameters for CO
2
!
",$
= &
1
() − )
0
)
3
+ &
2
() − )
0
)
2
+ &
3
() − )
0
) + &
4
TOPIC 1: Gas Hydrates
44
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
The alpha parameters
2
CO
0,
α
and
2
CO
1,
α
for
CO
2
are then found to be
368
.
0
2
CO
0,
=
α
752
.
0
2
CO
1,
=
α
The correction factor for CO
2
is then found from equation (1.5) as follows:
!
"
= $%
0,"
. )
*
+ %
1,"
-.
"
+ 1.000
(
)
000
.
1
0239
.
0
752
.
0
1061
.
0
368
.
0
2
CO
+
×
+
×
=
B
019
.
1
2
CO
=
B
The correction factor
k
B
for the hydrate dissociation pressure
accounting for
the non-hydrocarbon component
k
is defined as
!
"
=
Hydrate dissociation pressure of
real fluid
Hydrate dissociation pressure of hydrocarbon only fluid
Thus, the hydrate dissociation pressure of hydrocarbon fluid plus CO
2
is given
by
(
)
1661
019
.
1
1630
2
CO
=
×
=
+
hf
P
kPa
The N
2
multiplier
2
N
B
is calculated in the same
way using the constants in
Table 3 for nitrogen – this leads to the following results:
277
.
1
2
N
0,
=
α
091
.
1
2
N
1,
=
α
007
.
1
2
N
=
B
The total hydrate dissociation pressure, P, therefore becomes:
(
)
1673
007
.
1
1661
2
2
CO
=
×
=
+
+
N
hf
P
kPa
The experimental hydrate dissociation pressure at 280.55 K has been
measured at 1620 kPa.
………………………………………
..
TOPIC 1: Gas Hydrates
45
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
Figure 22 presents the experimental hydrate free zone for the gas condensate
together with the predictions of HWHYD (see statistical mechanical methods)
and the developed correlation.
Figure 22: Experimental and Predicted Hydrate
Stability Zone using
HWHYD Correlation
The above results are for a North Sea gas condensate fluid using the HWHYD
correlation, experimental data and a comprehensive thermodynamic model.
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