China Petroleum Processing and Petrochemical Technology
Scientific Research
2015, Vol. 17, No. 3, pp 32-38
June 30, 2015
Experimental Study on the Characteristics of CO2 Hydrate
Formation in Porous Media below Freezing Point
Zhang Xuemin1; Li Jinping 1; Wu Qingbai1, 2; Wang Chunlong1; Nan Junhu1
(1. Western China Energy & Environment Research Center, Lanzhou University of Technology, Lanzhou 730050;
2. Cold and Arid Regions Environmental Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000)
Abstract: Porous medium has an obvious effect on the formation of carbon dioxide hydrate. In order to study the characteristics of CO2 hydrate formation in porous medium below the freezing point, the experiment of CO2 hydrate formation was
conducted in a high-pressure 1.8-L cell in the presence of porous media with a particle size of 380 μm, 500 μm and 700 μm,
respectively. The test results showed that the porous medium had an important influence on the process of CO2 hydrate formation below the freezing point. Compared with porous media with a particle size of 500 μm and 700 μm, respectively, the
average hydrate formation rate and gas storage capacity of carbon dioxide hydrate in the porous medium with a particle size
of 380 μm attained 0.016 14 mol/h and 65.094 L/L, respectively. The results also indicated that, within a certain range of
particle sizes, the smaller the particle size of porous medium was, the larger the average hydrate formation rate and the gas
storage capacity of CO2 hydrate during the process of hydrate formation would be.
Key words: CO2 hydrate; formation rate; porous media; formation characteristics; gas storage capacity
1　Introduction
(4) CO2 replacement, which replaces the CH4 molecules
Naturally occurring gas hydrates in deposits have been
trapped in hydrate structure with CO2 molecules. Most of
the above-mentioned methods can change the temperature
or pressure of hydrate reservoir below the equilibrium
condition in order to bring about hydrate dissociation.
However, each method has its own merits and demerits to
some extent.
Moreover, it is considered that CO 2 replacement is a
potential method for methane recovery from the gas hydrate reservoir. And the replacement process between
CO2 and CH4 hydrate includes two critical processes of
CH 4 hydrate dissociation and CO 2 hydrate formation
simultaneously. Qi, et al.[2] experimentally investigated
the processes of exchange between CO2 and CH4 hydrate
using molecular dynamics simulation. The results have
confirmed that CH4 is released from hydrate and enter
into the gas phase by the replacement with CO2 and it will
spend a long time without the dissociation of methane
hydrate. Jung, et al.[3] experimentally studied the process
for replacement of methane from the gas hydrate with
found in many regions of the world such as in permafrost
regions and in seafloor sediments where the existing pressures and temperatures allow for thermodynamic stability
of the hydrate[1]. Because the gas hydrates contain a large
amount of methane gas, hydrates in those regions have
been considered to be a future energy resource having a
potential prospect. So the methods for efficient and safe
exploitation of natural gas hydrate have been attracting
more attention of researchers. The natural occurrence
conditions of the gas hydrate in sediments are always
below the freezing point in permafrost regions. Based on
the natural occurrence conditions of natural gas hydrate,
several feasible schemes for producing methane from
the hydrate reservoir have been put forward, such as:
(1) depressurization, which decreases the pressure of hydrate reservoir under the condition of phase equilibrium;
(2) thermal stimulation, which increases the temperature of reservoir above the phase equilibrium conditions
through injecting hot water or steam; (3) inhibitor injec-
Received date: 2015-05-09; Accepted date: 2015-08-24.
tion, which injects chemical additives such as methanol
Corresponding Author: Prof. Li Jinping, Telephone: +86-931-
or glycol to shift the P-T equilibrium of hydrate; and
2976332; E-mail: [email protected].
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Zhang Xuemin, et al. China Petroleum Processing and Petrochemical Technology, 2015, 17(3): 32-38
CO2. The results showed that the replacement process of
CO2 and CH4 hydrate occurred locally and gradually so
that the overall hydrate mass remains solid and no stiffness loss should be expected in the sediment. Deusner, et
al.[4] also studied the replacement of CH4-hydrate deposits
through injecting CO2 at different sediment temperatures
in the high-pressure vessel. The result showed that the
temperature and pressure had an obvious influence on the
replacement of CO2 and CH4 hydrate in porous media.
Furthermore, Yuan, et al.[5] investigated the characteristics of CO2 hydrate formation in porous media during the
process of replacement. Goel, et al.[6] found that CO2 was
preferentially sequestered over methane in the hydrate
phase. Lee, et al.[7] experimentally studied the replacement process of CO2-CH4 hydrate and found that new
CO2 hydrate formed on the surface of the CH4 hydrate
would wrap the CH4 hydrate and prevent the replacement
reaction[8-11]. However, the natural gas hydrate usually exists steadily in porous media at temperatures below the
freezing point. And the formation of CO2 hydrate and the
dissociation of CH4 hydrate are the two critical processes
of replacement between CO2 and CH4 hydrate. Therefore,
it is necessary to study the process of CO2 hydrate formation
in porous media at temperatures below the freezing point.
Many researchers have investigated the process of hydrate
formation in porous media. Handa, Stupin, et al.[12] experimentally demonstrated that the disassociation pressure of
CH4 and C3H8 hydrates in porous media with small pores
was higher than those in bulk. Uchida, et al.[13] observed
an equilibrium shift of CH4 hydrate in porous glass with
a pore radius of 100 Å, 300 Å, and 500 Å, respectively.
Besides, many other researchers reported the equilibrium
shift for hydrates in porous medium with a pore radius
smaller than 600 Å[14-16]. Dicharry, et al.[17] experimentally
studied the process of CO2 hydrate formation in porous
silica gel with pure water or surfactant solution. The results showed that, the smaller the nominal pore diameter
of porous medium was, the lower the temperature and the
higher the pressure for the shift of equilibrium conditions
sociation and gas production behavior.
Although there have been many researches on the process of hydrate formation and dissociation in porous
media, there are few focusing on the characteristics of
hydrate formation in porous media at temperatures below
the freezing point. In this paper, the experiment on CO2
hydrate formation was conducted in porous media with
different pore diameter at temperatures below the freezing point in a high-pressure cell with a volume of 1.8 L.
These results are expected to provide some useful information on the kinetic behaviors and formation characteristics in porous media at temperatures below the freezing
point, and can also provide an important theoretical guidance for the exploitation process of natural gas hydrate in
permafrost regions.
2　Experimental
2.1　Apparatus and reagent
The schematic diagram of the experimental apparatus is
shown in Figure1. The experimental apparatus consists
of a thermostatic bath equipped with a low-temperature
circulating cooler using alcohol as the cooling medium,
a high-pressure cell with a volume of 1.8 L which is
equipped with a 5.0-L buffer tank made of 316 L stainless steel the temperature of which is controlled by a lowtemperature circulating device, a gas cylinder, which
supplies carbon dioxide and controls the system pressure,
a vacuum pump for vacuumizing the high-pressure cell
before charging carbon dioxide, and a data acquisition
system including a temperature sensor (with a precision
of ±0.05 K), a pressure transducer (with a precision of
±0.01 MPa), a data acquisition system operated by Agilent 34970A, and a data logger system. Inside the highpressure cell, two temperature sensors are installed and
employed to measure the temperature of gaseous phase
and sediment of porous media during the experiment.
Carbon dioxide with a purity of 99.99% used in the ex-
developed a nu-
periments was purchased from the Lanzhou Lanheng Spe-
merical model to study the process of hydrate dissociation
cial Gas Co., Ltd. The quartz sand with a purity of 99.0%
and gas production in porous media by depressurization.
used in the experiment was provided by the Tianjin Yuanli
Their simulation results showed that the permeability
Chemical Co., Ltd. Nitrogen with a purity of 99.9% was
characteristics had a significant impact on the hydrate dis-
provided by the Zhejiang Tricyclic Chemical Reagent
[18]
of CO2 hydrate would be. Ruan, et al.
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Zhang Xuemin, et al. China Petroleum Processing and Petrochemical Technology, 2015, 17(3): 32-38
Figure 1　Schematic diagram of experimental apparatus
1—Gas cylinder; 2—Relief valve; 3-Valve; 4—Vacuum pump; 5—Thermostatic bath; 6—Valve; 7—Reaction cell; 8, 10—Valve;
9—Pressure transducer; 11— Buffer tank; 12—Temperature sensor; 13—Low temperature circulator; 14—Data acquisition; 15—Computer
Co., Ltd. The water used in the experiment was deionized
distilled water produced in the laboratory.
2.2　Experimental procedures
In the experiment, 450 mL of deionized water were added
into 1 L of porous medium made of clean quartz sand
with a particle size of 700 μm so that the deionized water
was distributed homogeneously in the quartz sand. The
mixture was subject to freezing immediately and adequately, while the water would become ice at the same
time. Then ice was crushed into powder with a certain
particle size under the protection of liquid nitrogen. After
that, the ice powder was added into the high-pressure cell
which was immersed in the alcohol bath, and the whole
process in the thermostatic bath was terminated and the
experiment was finished at the same time.
By using the same method, the process of CO2 hydrate
formation was conducted in a porous medium with a particle size of 380 μm. During the experiment, 420 mL of
deionized water were added into 1 L of the clean porous
medium with a particle size of 380 μm in the high-pressure cell. And other procedures were similar to those used
in the experimental process of CO2 hydrate formation using the porous medium with a particle size of 700 μm.
3　Results and Discussion
system was tested for air tightness under vacuum and then
3.1　Effects of porous media on process of CO2 hydrate formation
at a certain pressure, respectively. If the pressure variation
In order to investigate the influence of porous media on the pro-
in the high-pressure cell was less than 0.000 2 MPa/h, the
cess of CO2 hydrate formation below freezing point, the for-
device was considered to be in compliance with good air
mation experiment was conducted in porous media made of
tightness. After the air tightness testing, the high-pressure
quartz sand with different particle size. The variation in pres-
cell was purged with carbon dioxide and the procedure
sure and temperature in the course of CO2 hydrate formation
was repeated three times at least. Then CO2 was injected
is shown in Figure 2, Figure 3 and Figure 4, respectively.
into the high-pressure cell until its initial pressure reached
In Figures 2—4, the variation in pressure and tempera-
3.6 MPa and then the temperature of the thermostatic bath
ture shows the typical characteristics of carbon dioxide
was set at 273.15 K. When the temperature was stabilized
hydrate formation in porous media. The temperature and
at 273.15 K, it was regulated to 271.15 K and the reaction
system began to cool down. Then the variation in temper-
pressure decreased in the high-pressure cell with the continuous cooling of the bath. After a period of induction
ature and pressure was observed and recorded through the
time, the carbon dioxide hydrate would form and grow in
data acquisition device during the process of CO2 hydrate
porous media and the amount of CO2 would decrease with
formation. When the temperature and pressure changes
were negligible, it was considered that the process of
CO2 hydrate formation was completed. Then the cooling
time. The process of CO2 hydrate formation was an exo-
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thermic reaction and the heat of formation was released
when CO2 hydrate was formed in porous media. And then
Zhang Xuemin, et al. China Petroleum Processing and Petrochemical Technology, 2015, 17(3): 32-38
Figure 2　The curves of temperature and pressure
Figure 4　The curves of temperature and pressure
variations during the process of CO2 hydrate formation in
variations during the process of CO2 hydrate formation in
porous medium with a particle size of 700 μm
porous medium with a particle size of 380 μm
■—Pressure; ▲—Temperature of sediment;
■—Pressure; ▲—Temperature of sediment;
●—Temperature of free gas;
●—Temperature of free gas;
◆—Temperature of constant temperature bath
◆—Temperature of constant temperature bath
thereof. Finally, a phase equilibrium of CO2 hydrate formation was reached under the experimental conditions of
specified temperature and pressure.
It can be seen from Figures 2—4 that the pressure of
the system was influenced by the temperature variation
during the process of CO2 hydrate formation in porous
media. Because the hydrate formation process was an
exothermic reaction, the heat of formation was released,
whereas the temperature would fluctuate in the course of
Figure 3　The curves of temperature and pressure
variations during the process of CO2 hydrate formation in
porous medium with a particle size of 500 μm
■—Pressure; ▲—Temperature of sediment;
●—Temperature of free gas;
CO2 hydrate formation. Therefore, the pressure drop in
the system would slow down. It can be seen from Figures
2—4 that the curves of temperature variation showed irregular fluctuations over the time of reaction. When the
CO2 hydrate formation process continued to go on, the
◆—Temperature of constant temperature bath
variation in temperature weakened gradually with time.
it can be clearly seen that the temperature fluctuated from
pure water, the gas-liquid contact between CO2 and water
the two curves of the temperature and pressure variations
during the hydrate formation process. Meanwhile, the
pressure of the system gradually decreased as the temperature dropped with time. The amount of CO2 gas was
super-saturated through adsorption and the pressure of
the reaction system was affected by the amount of component in the gaseous phase. As the process continued to
take place, the pressure decreased gradually because of
the continued consumption of CO2 gas in the hydrate formation process. And the variation in temperature also fell
off with a decreasing amount of formation heat released
Compared with the process of CO2 hydrate formation in
was quite sufficient and the diffusion process of CO2 gas
was strengthened in the porous medium. Sun and Li, et
al.[19-20] drew up the similar conclusions during investigating the process of natural gas hydrate formation. Furthermore, this phenomenon could help to overcome the mass
transfer limitations usually observed during the experiments on hydrate formation in porous media.
3.2　Effects of particle size of porous media on rate of
CO2 hydrate formation
The experiment was conducted under the condition of a
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Zhang Xuemin, et al. China Petroleum Processing and Petrochemical Technology, 2015, 17(3): 32-38
constant volume adopted in the process of CO2 hydrate
formation. The rate of CO2 hydrate formation is defined
as the consumption rate of CO2. And it can be characterized by changes in the number of moles of CO2 gas in
the reaction system. Because the pressure variation is
influenced by the temperature of reaction system, it is
very difficult to describe the changes in moles of gaseous
phase in the reaction system. So the gas storage volume
and gas storage rate of CO2 are characterized by the ratio
of moles of CO2 converted into the CO2 hydrate versus
the moles of water. Thus the calculated value of the average formation rate of CO2 was based on the consumption
of CO2 gas during the experiment. And CO2 gas exists in
two forms in the reaction system, namely: the free CO2
in gaseous phase and the CO2 converted into the hydrate.
Hereby an assumption states that the number of moles
are expressed as ng and nh, respectively. And n stands for
a(Tc)=0.457 24 R2Tc2/Pc
(7)
b(Tc)=0.077 80 RTc/Pc
(8)
0.5
0.5
α =1+k(1-Tr )
k=0.374 64+1.542 26ω-0.269 92ω2
Tr=T/Tc
(9)
(10)
(11)
The amount of CO2 in the experiment was calculated by
the equation of state. In this work, it has been shown that
the particle size of porous medium has an obvious influence on the rate of hydrate formation to some extent. According to the experiment on CO2 hydrate formation in
porous media, the calculated average rate of CO2 hydrate
formation in the porous medium with a particle size of
380 μm was 0.016 14 mol/h, while it was 0.014 82 mol/h and
0.014 03 mol/h in porous medium with a particle size of
500 μm and 700 μm, respectively. The rate of CO2 hy-
able for nucleation of the hydrate. The reaction system
drate formation in the porous medium with a smaller particle size was obviously greater than that of porous media
with a larger pore size of 500 μm and 700 μm, respectively.
The results showed that the smaller the particle size was, the
larger the average rate of CO2 hydrate formation would be.
Compared with the pure water system, the gas-liquid con-
was in a flow state and CO2 gas was surplus in the hydrate
tact area between CO2 gas and water was even sufficient
formation system during the experiment. Since water was
present in a state of ice in the porous medium, there were
two forms of CO2, viz.: the free CO2 and the CO2 converted into hydrate in the reaction system.
Before the CO2 hydrate formation, the CO2 gas existed
mainly in the form of free CO2 in gaseous phase. After
the CO2 hydrate formation, there were two forms of CO2
in the system including the free CO2 gas and the CO2 that
was converted into the hydrate. In this case the following
equation applies:
n=ng+nh
(1)
At a certain temperature, ng is calculated according to the
equation of state (EOS) as shown below:
pV=ZnRT
(2)
and the diffusion process of CO2 gas was strengthened
the total number of moles of CO2 gas in the reaction system. However, the CO2 gas at first could diffuse into the
porous medium gradually and then it was converted into
CO2 hydrate when it reached a certain concentration suit-
V
ng =
Vm
P=
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(3)
in the porous medium, which could help to overcome
the mass transfer limitations usually observed in experiments with porous media. Furthermore, it could provide
larger specific surface area in the porous medium. And
the smaller the particle size of porous medium, the larger
the specific surface area, which could have greater gas
adsorption capacity of CO2 in the process of hydrate formation. Meanwhile, it could afford a greater driving force
for the hydrate nucleation and growth process, which was
consistent with Yang’s view on methane hydrate formation characteristics in porous media. So it is demonstrated
that the average rate of CO2 hydrate formation in porous
medium with a particle size of 380 μm was greater than
that in porous media with a particle size of 500 μm and
700 μm in the experiment, respectively.
RT
a (T )
−
V − b V (V + b) + b(V − b)
(4)
3.3　Effects of particle size of porous media on gas
storage capacity of CO2 hydrate
a=a(Tc)α(Tr,ω)
(5)
b=b(Tc)
(6)
In order to clarify the influence of porous medium on
the gas storage capacity of CO2 hydrate below the freez-
Zhang Xuemin, et al. China Petroleum Processing and Petrochemical Technology, 2015, 17(3): 32-38
ing point, the experiment of CO2 hydrate formation was
conducted in a porous medium made of quartz sand with
different particle size. In this experiment, the hydration
reaction was conducted under the condition of constant
volume in the porous medium during the process of CO2
hydrate formation. The volume of CO2 gas remained constant in the high-pressure cell. Gas consumption in a unit
time and the total amount of CO2 gas were calculated by
the equation of state of gas.
Figure 5　The curves of remaining amount of CO2 in the
process of CO2 hydrate formation in porous medium with a
particle size of 380 μm and 700 μm, respectively
■—Porous medium of 380 μm; ▲—Porous medium of 700 μm
The gas storage capacity of CO2 was characterized by the
consumed gas volume at the end of experiment. Figure 5
shows the remaining amount of CO2 gas measured in the
course of CO2 hydrate formation in the porous medium
with different particle size.
In this study, we found out that the particle size of quartz
sand used as the porous medium had an insignificant influence on the gas storage capacity of CO2 hydrate in the
porous medium. Then we can calculate the gas storage
volume of CO2 in the porous media with different particle size. It is indicated that the gas storage capacity of
hydrate is related to the particle size of porous media to
some extent. According to the experiment of CO2 hydrate
formation in porous media below the freezing point, the
gas storage capacity of CO2 hydrate was 65.094 L/L in
the porous medium with a particle size of 380 μm, while
it was 59.854 L/L and 56.605 L/L in the porous medium
with a particle size of 500 μm and 700 μm, respectively.
The results showed that the smaller the particle size, the
smaller the pore diameter of the porous medium and
the larger the gas storage capacity of CO2 hydrate in the
porous medium, which was largely dependent on the
physical parameters of porous medium. Since the porous
medium with different particle size had different specific
surface area, so it would result in different adsorption
capacity of CO2 gas. In addition, the smaller the particle
size of porous medium was, the bigger the specific surface area would be, which was beneficial to the process
of gas adsorption in the system of hydrate formation. And
more amount of CO2 gas could penetrate into quartz sand
used as the porous medium during the process of hydrate
formation. In Figure 5, the gas consumption of CO2 in the
porous medium with a particle size of 380 μm was greater
than that in porous media with a particle size of 500 μm
and 700 μm, respectively. Furthermore, this phenomenon
could accelerate the process of mass transfer in the porous
media and provide favorable conditions for realizing the
process of hydrate nucleation and growth. So the gas storage capacity of CO2 in the porous medium with a particle
size of 380 μm was greater than that in porous media with
a particle size of 500 μm and 700 μm, respectively. Therefore, it was concluded that, within a certain particle size, the
smaller the particle size of the porous media was, the larger
the gas storage capacity of CO2 hydrate in porous media would be.
4　Conclusions
In this communication, the characteristics of CO2 hydrate
formation has been investigated in porous media with a
particle size of 380 μm, 500 μm and 700 μm, respectively,
in an 1.8-L high-pressure cell operated at a temperature
of below the freezing point. Based on the experimental
results, the following conclusions were drawn up:
(1) In the porous medium with a particle size of 380 μm,
the gas storage capacity and the average hydrate formation rate were the maximum among the three kinds of
porous media, and each volume of CO2 hydrate might
contain 65.094 volumes of CO2 gas under the standard
condition, with the average hydrate formation rate reaching up to 0.01614 mol/h.
(2) In the porous medium with a particle size of 700 μm,
the gas storage capacity and average hydrate formation
rate were the minimum among the three kinds of porous
media, and each volume of CO2 hydrate might contain
56.605 volumes of CO2 gas under the standard condition,
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Zhang Xuemin, et al. China Petroleum Processing and Petrochemical Technology, 2015, 17(3): 32-38
with the average formation rate reaching 0.014 03 mol/h.
(3) Within a certain range of particle size between 380 μm
and 700 μm, the smaller the particle size of porous medium was, the faster the average rate of hydrate formation
and the larger the gas storage capacity of CO2 in the hydrate would be.
Acknowledgements: This work was financially supported by
the Natural Science Foundation of China (No. 51266005) and
with CO2 in clathrate hydrate: observations using Raman
spectroscopy[C]. Proceedings of the Fifth International
Conference on Greenhouse Gas Control Technologies.
CSIRO Publishing: Collingwood, Australia, 2001: 523-527
[11]　Bai D, Zhang X, Chen G. Replacement mechanism of
methane hydrate with carbon dioxide from microsecond
molecular dynamics simulations[J]. Energy & Environmental Science, 2012, 5(5): 7033-7041
the Science and Technology Research Key Project of the Minis-
[12] Handa Y P, Stupin D Y. Thermodynamic properties and
try of Education (No. 1106ZBB007), the Hongliu Outstanding
dissociation characteristics of methane and propane hy-
Talent Program of LUT (No. Q201101) and the Open Fund of
drates in 70-Å-radius silica gel pores[J]. The Journal of
Natural Gas Hydrate Key Laboratory, Chinese Academy of Sci-
Physical Chemistry, 1992, 96(21): 8599-8603
[13]　Uchida T, Ebinuma T, Ishizaki T. Dissociation condition
ences (No. y007s3).
measurements of methane hydrate in confined small pores
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