TOPIC 1: Gas Hydrates
39
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
The advantages and of predictive techniques can be summarised as follows:
calculations are generally fast and little time is required for determining the
hydrate
stability zone; they provide significant savings on manpower and
experimental equipment; they provide considerable
flexibility in simulating
various production scenarios; predictive techniques could also be used to
design remedial work.
The disadvantages of predictive techniques can be summarised as follows:
they are limited to the accuracy of the technique and the expertise
to represent
the fluid system; generally some approximations are necessary which reduces
the accuracy of the results.
1.7.3 Gas Gravity Method
With this method, given the gas gravity and the system temperature, the
hydrate stability pressure can be determined. This
is achieved by drawing a
vertical line from the system temperature and finding the intersection with the
appropriate curve corresponding to the gas gravity – at this intersection a
horizontal line is drawn to determine the hydrate stability pressure. Figure 21
shows the basic graphical method that was developed by Katz.
Figure 21: Gas Gravity Chart for Hydrate Prediction (Katz, 1945)
TOPIC 1: Gas Hydrates
40
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
The above method has several limitations.
It was developed based on a
limited number of typical natural gases and as a result there are significant
deviations when the concentrations of non-hydrocarbon gases are high. Also,
it can only be used for gaseous systems and could result in significant error,
when applied to systems containing liquid hydrocarbons.
The Centre for Gas Hydrate Research at Heriot-Watt University developed a
correlation which eliminates most of the above limitations:
! = #$%{[(
1
× (, + (
2
)
−3
+ (
3
. 3
4
+ (
4
. 3
4
2
+ (
5
] × 8 + (
6
(, + (
7
)
−3
+ (
8.
. 3
4
+ (
9
. 3
4
2
+ (
10
}
………………………
..
……
..(1.2)
Where,
P
is the hydrate dissociation pressure (kPa).
T
is the system temperature (K).
γ
is the
specific gravity of the hydrate forming hydrocarbons.
The gas gravity is defined as
a
h
M
M
=
γ
………………………………………………
.(1.3)
Where,
h
M
is the molecular weight of hydrate forming hydrocarbons.
a
M
is the molecular weight of air (
=
a
M
28.966 g/mol).
i
C
are various c
onstants - see Table 2 below.
The molar ratio
m
F
between the non-hydrate forming hydrocarbons (subscript
“
nh
”
) and the hydrate forming hydrocarbons (subscript “
h
”) in the fluid:
h
nh
m
f
f
F
=
………………………………………………
(1.4)
Methane, ethane, propane, and butanes are regarded as
h
hydrocarbons and
pentanes and heavier components are regarded as
nh
compounds.
Table 2: Constants for HW Correlation
TOPIC 1: Gas Hydrates
41
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
The effect of nitrogen and carbon dioxide on the hydrate free zone for real
fluids was correlated (using maximum concentrations of 15 mole% and 10
mole%, respectively) by the following expression:
!
"
= $%
0,"
. )
*
+ %
1,"
-.
"
+ 1.000
……………
..
…
.(1.5)
Where
k
B
is the correction factor for the hydrate dissociation pressure
accounting for the non-hydrocarbon component
k
and is defined as follows:
!
"
=
Hydrate dissociation pressure of
real fluid
Hydrate dissociation pressure of hydrocarbon only fluid
………………
.....(1.6)
and
k
f
is mole fraction of the non-hydrocarbon component
k
in the fluid.
Non-hydrocarbon components
k
are usually either nitrogen or carbon dioxide
(
m
F
was defined earlier). Also, the alpha parameter
k
i
,
α
can be expressed as
!
",$
= &
1
() − )
0
)
3
+ &
2
() − )
0
)
2
+ &
3
() − )
0
) + &
4
………………………
.(1.7)
The system temperature is represented by
T
(K), while
0
T
= 273.15 (K). The
four coefficients, corresponding to each alpha parameter
k
i
,
α
, are listed in the
four columns of Table 3.
Table 3. Constants used to Determine Alpha
k
i
,
α
Parameters
Advantages of the New Correlation are:
•
It applies across the range of fluid systems, from lean gas to black oil.
•
Only specific gravity and the concentration
of hydrate forming
compounds are required.
•
The effect of non-hydrocarbon gases can be included.
•
It covers a wide range of temperature and concentrations.
•
The estimated error is around 1 K.
TOPIC 1: Gas Hydrates
42
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
The HW correlation is applicable over the range of parameters shown in Table
4.
Table 4: Parameters Range of HW Correlation
Dostları ilə paylaş: