Journal of Natural Gas Chemistry 16(2007)293–300
Article
The Possibility of Wax Formation in Gas Fields: a Case Study
Z. Jeirani1 ,
A. Lashanizadegan1 ,
Sh. Ayatollahi1 ,
J. Javanmardi2
1. School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran;
2. School of Chemical Engineering, Shiraz University of Technology, Shiraz, Iran
[ Manuscript received March 2, 2007; revised July 10, 2007 ]
Abstract: Natural gas production from a gas reservoir (Reservoir A) located in the south of Iran, presents
solids deposition during processing because the condensate contains suspended and dissolved solids. Solids
deposition occurs not only in the transportation lines from the wells to the separators but also in the
various operating units of gas streams and condensate stream. In this study, the multisolid-phase model
has been used to predict the wax precipitation from gas and gas condensate fluids. The properties of gas
and liquid phases are described using the Soave-Redlich-Kwong (SRK) equation of state. The model is
then used to predict the possibility of the wax formation in Reservoir A gas facilities, located at the south
of Iran. Solid deposition which occurred in the various streams of that facility confirmed the calculated
results. Finally, the wax appearance temperature (WAT), the weight percent of wax formation and the
effects of pressure and temperature on the wax formation were also predicted.
Key words: wax precipitation; fluid phase equilibria; multisolid model; gas condensate
1. Introduction
Gas and gas-condensate fluids contain a very wide
range of hydrocarbons from methane and ethane, to
the components as heavy as C40 or C50 or much higher
hydrocarbons. It is believed that the broad volatility
and melting-point range of these hydrocarbon components will cause the formation of solid phases in
response to thermodynamic state changes. Hydrocarbon solid precipitations such as wax and asphaltene in
the gas and oil production facilities and pipelines are
undesirable because it may plug them. Wax precipitation can occur from gas condensate, light oil, and
heavy oil fluids at temperatures as high as 338.7 K.
The wax precipitation from crude has been extensively studied; however, the precipitation from gas
condensates has been investigated recently by some
researchers.
It is generally accepted that almost any form of
solid deposition during the production and processing
of oil and gas streams has unfavorable economic and
∗
operational consequences [1]. Typically, these precipitating solids can be any one of the pure solids, hydrates, waxes, asphaltenes, or scales and they may
even co-precipitate.
Petroleum fluids contain heavy paraffins that
may form solid wax phases at low temperature in
the pipelines and hydrocarbon production facilities.
The problems caused by wax precipitation such as
decreasing production rates, increasing power requirements, and failure of facilities, are a major concern
in the production and transportation of hydrocarbon
fluids. In order to prevent this process, which is linked
to changing pressure and temperature conditions and
fluid composition, it is essential to be able to predict
the phase behavior of the reservoir fluid.
Wax precipitation is an old problem for which attempts have been made to develop a thermodynamic
description. Modeling solid deposition allows the engineer to carry out a variety of tasks from screening
for potential problems through to possible remediation should they occur. Used in conjunction with field
Corresponding author. Tel: 0098-711-6275678; Fax: 0098-711-6287294; E-mail: shahab@shirazu.ac.ir
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Z. Jeirani et al./ Journal of Natural Gas Chemistry Vol. 16 No. 3 2007
experience and appropriate laboratory measurements
models can be used to predict the conditions under
which solids may form, on the basis of a straightforward fluid analysis of the gas, condensate or oil, and
to consider and compare possible corrective actions
such as altering the operating conditions or introducing chemical inhibition.
The cloud point temperature or wax appearance
temperature (WAT), where wax is first detected on
cooling, is commonly measured in the laboratories.
However, some of the published methods for describing wax precipitation are in poor agreement with
the experimental data; they tend to overestimate the
amount of wax at temperatures below the cloud-point
temperature [2].
Generally, wax precipitation models are divided
into two categories: The first approach is on the basis
of the flash-type calculations, and the second one from
direct minimization of the Gibbs free energy. In the
flash-type calculations, an Equation of state (EoS) is
applied for vapor-liquid equilibria (VLE) prediction.
In addition, an activity model, on the basis of either
regular solution theory or polymer solution theory is
used for solid-liquid equilibria (SLE) [3−6]. The VLE
and SLE models use two types of thermodynamic approaches to describe the non-ideality of the liquid
phase: an EoS for VLE and an activity coefficient
model for SLE. It is found that these types of calculations, however, are thermodynamically inconsistent.
And it was suggested by several researchers that the
latter approach should employ an EoS for all three
phases to overcome the discrepancy of vapor-liquidsolid properties predictions [2,7−8]. Because of the
uncertainty of heavy components’ properties, some
adjustable parameters and a tuning function were introduced by Mei et al, and also the influence of the
heat capacity difference between the liquid and the
solid on the solid precipitation was taken into consideration [9]. Vafaie-Sefti et al. presented a modified
multisolid phase model for predicting phase equilibria for oil mixtures and the possibility of wax formations [10]. Zuo et al. developed a solid-solution model
using the Poynting correction factor to estimate the
solid fugacity [11]. Ji et al. presented a new thermodynamic model for the wax weight percent prediction that had validated the model predictions were
against the results obtained from the measurements
of wax disappearance temperature (WDT) data for a
number of binary and multi-component systems [12].
The wax precipitation at temperatures as high as
338.7 K may occur for both crude oils and gas con-
densates [13]. The wax precipitation from crude has
been extensively studied; the precipitation from gas
condensates, however, has been recently investigated
by some researches. Leontaritis pointed out the possible damage caused by the near-wellbore formation as
a result of wax deposition from gas condensates [14].
Moreover, the authors indicated that the gas condensate containing high-carbon-number waxes may start
precipitating at the dew point pressure. This phenomenon is caused by the existence of the wax deposition
envelope (WDE) as suggested by Leontaritis [14].
One of the most interesting features of wax precipitation from the gas condensate fluids was studied in detail by Nichita and Firoozabadi [15]. It was
found that the precipitated wax phase can exhibit retrograde phenomena similar to that in the gas condensates. This model considers for both the Poynting correction term and the solid-state phase transitions [15].
Reservoir hydrocarbon fluids commonly consist of
both liquid and vapor phases at pipeline conditions.
As mentioned earlier, the vapor-liquid equilibrium is
commonly described using an equation of state. In order to ensure consistency in description of the liquid
phase, the wax thermodynamic model must also use
an equation of state for calculating fugacity in the liquid phase. SRK EoS [16] and PR EoS [17] are popular
for calculating fugacities in vapor-liquid systems.
Critical properties are needed to perform vaporliquid-solid equilibria (VLSE) using the cubic EoS. It
is almost impossible to measure directly critical properties of long-chain n-paraffins, because of their thermal decomposition at high temperatures. As a result,
different correlations have been suggested in the literature for an estimation of the critical properties and
acentric factors of these compounds [18−19] and the
estimated values can differ from each other considerably. On the other hand, the differences between the
predicted and the experimental values regarding the
wax precipitation in light hydrocarbon reservoirs are
still very high.
National Iranian Oil Company (NIOC) is producing 40 million m3 /day of natural gas and 7000 bbl/day
of condensate from a gas field at the south of Iran.
The separated gas is transported through a 330 km
pipeline to the Marun oil field for pressure maintenance. Because of the formation of sludgy-deposited
materials in the pipelines and separators, it is worthy to study the possibility of the wax formation in
this gas condensate reservoir. In this study, the wax
precipitation from the gas condensate fluids is studied
using the multisolid phase model. The correlations for
295
Journal of Natural Gas Chemistry Vol. 16 No. 3 2007
Paraffinic-Naphtenic-Aromatic (PNA) species given
by Riazi and Al-Sahhaf and Pan et al. are used to predict the critical properties [18,8]. The gas and liquid
properties are also described using the SRK EoS. The
correlations suggested by Riazi and Al-Sahhaf for critical properties, has been recommended in conjunction
with the SRK EoS because of the consistency between
the optimized values of m for heavy paraffins [12].
2. MultiSolid phase model
Waxes are more difficult to understand than pure
solids because they are complex mixtures of solid hydrocarbons that freeze out of crude oils if the temperature is low enough. Waxes are mainly formed
from deposited normal paraffins but isoparaffins and
naphthenes are also present and some waxes have an
appreciable aromatic content. It is known that the
paraffins are the major components in the wax deposition and their values are greater than the values of other components in the solid phase, and also,
paraffins are the first components that come out from
the precipitates. Multisolid model is based on the precipitation of certain heavy components of crude oil
and gas condensate with average properties assigned
to each fraction [2].
According to this model, solid phases, which are
formed from different pure components, are in equilibrium with each other and are in equilibrium with
liquid and vapor phases. Figure 1 presents the vaporliquid-solid phases in this model. This model assumes
the precipitated species from the crude oil and gas
condensate which consist of pure components that do
not mix with other solid phases after precipitation [2].
this expression, will precipitate, whereas those that
do not will only be present in the liquid and vapor
phases.
The equilibrium criteria at known temperature
and pressure will result in,
s
fiv = fil = fi,pure
(2)
where fi is the fugacity of component i.
For non-precipitating components, the equilibrium condition is,
fiv = fil
(3)
In order to calculate the fugacity of each component in pure solid state, the following equation is
used [20]:
∆hfi
Tif
s
l
−
1−
fpure,i (P, T ) = fpure,i (P, T ) × exp
T
RTif
∆cpi
Tf
∆cpi
Tf
ln i
1− i −
R
T
R
T
(4)
where ∆hfi is the molar enthalpy of fusion, Tif the fusion temperature, ∆cpi the heat capacity of fusion for
component i, and T is the temperature of the systems.
2.1. Fugacity of pure liquid
Fugacity of each component in pure liquid state
is obtained from the following equation:
l
fpure,i
= φlpure,i P
(5)
The fugacity coefficient, φlpure,i is found using an
equation of state for component i at the temperature
and pressure of the system. In this study SRK EoS is
used. Binary interaction coefficients are the key parameters of this calculation found from the following
equation [20]:
1/6 1/6
EoS
= 1.0 −
kij
The number and the nature of precipitating components are obtained from a phase stability analysis
as follows [2]:
fi (P, T, zi ) −
0.0
1/3
1/3
vci + vcj
(6)
where vc is critical volume estimated from the following expression [2]:
Figure 1. Multisolid phase for wax precipitation
s
fi,pure
(P, T )
2vci vcj
(1)
where fi (P, T, zi ) is the fugacity of component i, having feed composition z. The components that fulfill
vci = (RTci /Pci )(0.290 − 0.085ωi)
(7)
2.2. Fusion properties
Fusion properties, used in Equation (4), are related to the solid precipitation or dissolution at
different conditions.
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Z. Jeirani et al./ Journal of Natural Gas Chemistry Vol. 16 No. 3 2007
2.2.1 Fusion temperature, Tif
For normal alkanes the following equation has
been suggested by Won [3]:
Tif = 374.5 + 0.02617Mwi −
20172
Mwi
(8)
For aromatics and naphthenes, the following
equation has been used by Lira-Galeana et al. [2]:
Tif = 333.5 − 419 exp(−0.00855Mwi)
(9)
2.2.2 Enthalpy of fusion, ∆hfi
For normal alkanes, naphthenes, and aromatics
the correlations suggested by Won, Lira-Galeana et al,
and Pan et al. have been used, respectively [2−3,8].
∆hfi
0.1426MwiTif
(10)
∆hfi = 0.05276MwiTif
(11)
∆hfi = 11.2Tif
(12)
=
2.2.3 Heat capacity of fusion, ∆cpi
For all components, the correlation suggested by
Pedersen et al. has been used [6].
∆cpi = 0.3033Mwi − 4.635 × 10
−4
Mwi T
To estimate the critical properties of light hydrocarbons, Riazi and Al-Sahhaf correlation have been
used in this study [18].
(14)
The critical pressures of components with molecular weight higher than 300 g/mol are, however, estimated by the following equation [8]:
Pc = A − B exp(−CMw )
ln ω = −36.1544 + 30.94Mw0.026261 (Mw 800)
(16)
For components with molecular weight greater
than 800, the acentric factor of 2 has been used.
The acentric factors for paraffins and naphthenes have
been estimated from Riazi and Al-Sahhaf correlations [18].
2.4. Crude oil systems
Pedersen et al. have provided extensive data
on wax-formation behavior of five petroleum mixtures [6]. The presented data are widely used by other
authors in different research works on wax precipitation [2,6,7,10].
Pan et al. reported the compositions of three
other petroleum mixtures. [8]. These systems are also
used by Vafaie-Sefti et al. to determine the amount of
the precipitated wax and the effects of pressure [10].
3. Results and discussion
(13)
2.3. Critical properties and acentric factor
ln(θ∞ − θ) = a − bMwc
The critical temperatures for all hydrocarbon components (aromatics, naphthenes, paraffins)
are calculated from Riazi and Al-Sahhaf’s correlations [18].
The following correlations [8] is used to estimate
acentric factor of aromatic components
(15)
where the constants A, B, and C are presented in Table 1.
Table 1. The values of the parameters A, B,
and C in Equation (15)
Coefficient
Paraffins
Naphthenes
Aromatics
A
0.67909
2.58854
4.85196
B
−22.179
−27.6292
−42.9311
C
0.0028417
0.0044951
0.0056193
Figures 2 shows the predicted wax weight percent for the oil systems given by Pedersen et al. and
Pan et al. [6,8]. The Figure indicates that the predictions obtained from the multisolid phase model are in
good agreement with the real behaviors of petroleum
mixtures. At a given temperature, the wax weight
percent is calculated for one mole of feed using the
following equation:
Wax weight percent =
Ns
=
j
total precipitated mass
× 100
mass of feed oil
Mwj
N
i
Sj
F
× 100
Mwi zi
(17)
where N and Ns are the number of the components
and the solid phases which is determined from Equation (1), respectively. Sj is the moles of solid phase j
and F is the feed mole numbers. Mwi and zi stand for
the molecular weight and mole fraction of component
i, respectively.
Journal of Natural Gas Chemistry Vol. 16 No. 3 2007
297
Figure 2. The weight percent of the precipitated solid as a function of temperature for dif ferent oils
(a) No. 1, (b) No. 4, (c) No. 5, (d) No. 6, (e) No. 10, (f) No. 11, (g) No. 12, (h) No. 15
To compare the results with the experimental
data and the other models, the wax formation weight
percent as a function of temperature for different systems given by Pedersen et al. and Pan et al. [6,8]
have also been shown in Figure 2. It should be noted
that the numbers in this Figure caption are the mixture numbers in references [6,8]. As can be seen, the
results of this study are in good agreement with the
298
Z. Jeirani et al./ Journal of Natural Gas Chemistry Vol. 16 No. 3 2007
experimental values. The over estimation found in
other works are possibly caused by the correlations
used in their models.
In summary, the correlations and the methods
used in this study for the description thermodynamic
properties of different phases have been given as follows:
1. As mentioned earlier, the correlations suggested by Riazi and Al-Sahhaf for the critical properties, has been recommended in combination with SRK
EoS [12]. Therefore, the SRK EoS is used here to calculate the fugacity of the components in both liquid
and vapor phases.
2. Binary interaction coefficients, as the key parameters of VLE calculations, are also estimated using
equation (6) [20].
3. The same equation of state (SRK) is used to
calculate the fugacities of all components in the vapor and liquid phases as well as the fugacities of the
pure liquids, used in Equation (4) for calculation of
the fugacities of solid phases. This causes the thermoWAT error percent =
dynamic consistency in the equilibrium calculations.
4. PNA analysis, as a very important parameter
in the wax calculations, is used here to calculate the
average properties of the oil fractions.
5. The correlations proposed by Pan et al. [8]
have been used to estimate the fusion and critical
properties as well as the acentric factors of each fraction.
6. The heat capacity difference between the solid
and liquid phases has been taken into account to improve the calculation of solid phase fugacities.
It should be noted that no adjustment parameters
have been used in this work.
As can be seen in Table 2, the calculated cloud
point temperatures are compared with the experimental data and other works. It has been found that there
is a very close agreement with the experimental data.
The good agreement is more obvious in Table 3 which
gives the error percent of WATs. This parameter is
calculated using the following equation:
calculated WAT − experimental WAT
× 100
experimental WAT
(18)
Table 2. The wax appearance temperature (K) at P =1 bar for dif ferent systems given by Pedersen et al. and
Pan et al. [6,8]
Wax appearance temperature (K)
Oil
No.
Exp.
Pedersen et al.
Lira-Galeana et al.
Pan et al.
Vafaie-Sefti et al.
This
(1991) [6]
(1996) [2]
(1996) [8]
(2000) [10]
work
1
304.15
303
305.9
−
−
303.9
4
322.0
−
−
322.0
322.0
322.0
5
315.0
−
312.4
312.8
312.5
314.2
6
320.0
−
−
319.0
317.65
319.6
10
314.15
293.15
316.0
−
321.0
314.1
11
295.15
311.15
299.3
−
−
294.7
12
305.15
279.15
301.2
−
311.0
304.2
15
308.15
283.15
309.5
−
310.0
307.8
Table 3. The error percent of the wax appearance temperature (K) at
P =1 bar for dif ferent systems given by Pedersen et al. and Pan et al. [6,8]
Oil
No.
Wax appearance temperature error percent(K)
Pedersen et al.
Lira-Galeana et al.
Pan et al.
Vafaie-Sefti et al.
This
(1991) [6]
(1996) [2]
(1996) [8]
(2000) [10]
work
1
−0.38
0.58
−
−
−0.08
4
−
−
0.00
0.00
0.00
5
−
−0.83
−0.70
−0.79
−0.25
−0.12
6
−
−
−0.31
−0.73
10
−6.68
0.59
−
2.18
−0.02
11
5.42
1.41
−
−
−0.15
12
−8.52
−1.29
−
1.92
−0.31
15
−8.11
0.44
−
0.60
−0.11
Journal of Natural Gas Chemistry Vol. 16 No. 3 2007
4. Case study of gas f ields
Natural gas produced from gas reservoirs A located at the South of Iran is used for the gas injection
into the oil fields for pressure maintenance. This plant
is designed for producing 40 million m3 /day of natural
gas and 6500−8000 bbl/day condensate using a refrigeration process to treat the gas and condensate to
meet the hydrocarbon dew point specifications. The
dry gas is then shipped into Khozestan for gas injection.
Natural gas production from this gas field
presents unique problems related to solids deposition
during processing because the produced fluids (gas
and gas condensate) contain suspended and dissolved
solids. Solids deposition occurs not only in the plant
inlet separators but also in the various operating units
in gas streams and gas condensate streams. To address the solids problem at the plant, a large set of
available data as well as stream properties were employed to find the possibilities of wax production using the results of this study. Table 4 illustrates the
hydrocarbon composition of gas reservoir A.
Table 4. The hydrocarbon composition of the
gas reservoir A
Component
Mol%
Component
Mol%
N2
5.453500
NC10
0.052745
CO2
1.297500
NC11
0.047765
H2 S
0.007000
NC12
0.037810
C1
89.766400
NC13
0.029835
C2
1.453000
NC14
0.022865
C3
0.430100
NC15
0.012915
IC4
0.114500
NC16
0.013905
NC4
0.172200
NC17
0.008935
IC5
0.082630
NC18
0.008938
NC5
0.064710
NC19
0.006948
IC6
0.000000
NC20
0.022780
NC6
0.128350
He
0.000000
IC7
0.000000
H2 O
0.442200
NC7
0.157250
Benzene
0.000000
IC8
0.000000
Toluene
0.000000
NC8
0.092575
Ethyl Benzene
0.000000
IC9
0.000000
Propyl Benzene
0.000000
NC9
0.072655
Total
100.000000
IC10
0.000000
Figure 3 shows the wax precipitation as a function of temperature for this composition data. It can
be seen that the wax precipitation starts at 305.8 K
299
under atmospheric pressure, which is far above the
local temperature during winter. Therefore, the precipitated waxes are identified as the major contributor to the solids problem in this plant. In addition,
the predicted values are confirmed by the field observation. Solid precipitation has been reported in the
transmission lines and the storage tanks of gas condensates in that field.
Figure 3. The weight percent of the precipitated
solid as a function of temperature for the
gas reservoir A
5. Conclusions
A multisolid-phase model, on the basis of the SRK
EoS to calculate the fugacities of different phases, is
used to predict the WAT and the percentage of wax
deposition. The model shows significant improvement
for both the WAT and percentage of wax deposition
calculations when the results thus obtained were compared with the experimental data.
The correlations, suggested by Riazi and AlSahhaf for critical properties, have been used in combination with the SRK EoS. These parameters lead
to consistency between the optimized values of m for
the heavy paraffins.
A case study of solid precipitation in a gas field
in the south of Iran is investigated using the present
model to predict the wax precipitation. The results
are confirmed by the field observation.
Acknowledgements
The authors wish to thank the Research and Development Directorate of the National Iranian Oil Company
for supporting this project and the South Zagros Oil and
Gas Producing Company for their very valuable help in
this regard.
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Z. Jeirani et al./ Journal of Natural Gas Chemistry Vol. 16 No. 3 2007
Nomenclature
References
f
P
T
z
h
cp
R
v
kij
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Mw
S
V
F
N
Ns
x
y
K
fugacity, bar
pressure, bar
temperature, K
feed composition
enthalpy, cal/mol
heat capacity, cal/(mol·K)
gas constant, cal/(mol·K)
molar volume, cm3 /mol
binary interaction coefficient between components i and j
molecular weight, g/mol
moles of solid phase, mol
moles of vapor phase, mol
feed mole numbers, mol
number of the components
number of solid phases
mole fraction of liquid phase
mole fraction of vapor phase
equilibrium coefficient
Superscripts
v
l
s
f
vapor phase index
liquid phase index
solid phase index
fusion point index
Subscripts
i, j
component index
pure, i pure state of component i
c
critical property
Greek letter
∆
φ
ω
θ
difference operator
fugacity coefficient
acentric factor
property operator
sum operator